It is Saturday afternoon and hundreds of tourists have come to snap selfies in front of Big Ben and to admire Westminster Abbey.

Jaime van Gastel, 52, however, is watching the crowds – not the architecture. 'There!' he says urgently. 'The man in white trousers. He's a pickpocket.'

He is pointing across the road, a good 40 yards away, at a man walking along Broad Sanctuary, a short street that leads to the main entrance of Westminster Abbey.

I am baffled: the man looks like a neatly dressed sightseer.

Van Gastel starts walking briskly, tracking him from the other side of the road. I am in hot pursuit and confused as to how he knows the man is a crook.

'Normal people, tourists, just walk around relaxed,' he says. 'He was stopping and looking nervous.' Van Gastel darts across the road, dodging traffic, to get closer.

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I follow and watch as van Gastel gets out his camcorder to record the white-trousered man walking towards the west door of the Abbey. The man appears to be following a Chinese woman, who has a Chanel quilted handbag dangling at her side.

Suddenly, as bold as brass, the man reaches forward, opens the flap of the handbag and darts his hand into it, not once, not twice, but three times – all while walking behind the victim. It could not be clearer: he is trying to steal something from the bag.

At that moment, van Gastel shouts: 'Pickpocket!' at the top of his voice. 'Why are you stealing? Look out: pickpockets!'

He films the man and a woman, who appears to be his accomplice.

She covers her face with her cardigan but the man saunters calmly away. At no point do they deny his accusations, nor question why he is making such a hullabaloo.

Van Gastel has – once again – stopped a pickpocket in his tracks. That's because his day job, in Amsterdam, is as a security guard for Uniqlo, the clothing company.

His weekend hobby, however, is going around the cities of Europe trying to stop or deter street thieves – recording it all on his camcorder with the help of his girlfriend Anne-Marie de Wit, 45.

In the Netherlands, he goes by the nickname 'Boevenspotter' or 'Crook spotter'.

'London is unbelievable. Unbelievable!' says van Gastel, still out of breath and smiling from his triumph. 'Normally, in Amsterdam, we spend all day and we catch one. But we came to London in August and in the first half hour, we caught the first one.'

He claims to have videoed 23 individuals over two days in August. When he posted his footage online, it went viral.

The Mail's alarm at the scale of the problem was – to be frank – mingled with a little scepticism about whether there really were so many crooks on the streets of London. And so we invited him back to spend an afternoon watching him in action. Sure enough, we'd only been going 20 minutes when he spotted his first attempted snatch.

According to the Metropolitan Police, crime overall in London has not dramatically worsened over the past year, increasing a modest 3.5 per cent between September 2023 and September 2024.

Pickpocketing, however, appears to be skyrocketing in the capital. The Met does not have a specific category called 'pickpocketing', but it does have something it calls 'thefts from the person', which includes pickpocketing as well as phone-snatching, and this has shot up by an eye-popping 43.2 per cent in the same period, with 101,159 offences recorded.

And this figure accounts only for crimes that victims have bothered to report. The total number of 'thefts from the person', estimated by the National Crime Survey – which bases its findings on a representative survey of members of the public – shows that just under four in ten such incidents are reported. Crime experts point out that many of us now routinely walk around with thousands of pounds worth of kit.

'Holding a phone on the street, carrying a laptop in a bag, listening to music on an expensive pair of headphones, have all become part of our daily routine,' says Danny Shaw, a policing expert.

'Thieves have cottoned on to this. There's big money to be made from selling on digital devices, and a lucrative illegal market for phones, in particular.'

Last month, a security company called Get Licensed crunched the Office for National Statistics data and found the country's top-ten pickpocket hotspots were all in London, with Westminster – which covers the centre of the capital – being the worst area in the country, with 13,320 reports of pickpocketing per 100,000 people. This is an astonishingly high rate. In the safest area of the country, North Kesteven in Lincolnshire, the rate is just three reports per 100,000.

There is a simple reason for this: Westminster is where many major tourist destinations are sited, including Big Ben, the West End and Buckingham Palace.

The number of tourists visiting the capital has surged over the past year – overtaking pre-pandemic levels – and many of them carry not just pricey phones but also a lot of cash. 'Asian people are the number-one target because they don't like credit cards. They come over here with lots of cash,' says van Gastel.

He has often witnessed criminals take the stolen cash straight to bureaux de change so they can transfer the money into their bank accounts, making any crime almost untraceable. Van Gastel adds: 'Why does London have a problem? The police do nothing. The crooks know they will not be stopped. We can confront them, but they just walk away.'

Shaw partly agrees. 'If a criminal steals a phone, headphones or laptop without using, or threatening, violence they'll probably end up with a community sentence or a short spell in prison,' he says. 'And that's assuming they're arrested and convicted, which itself is very unlikely. Fewer than 6 per cent of thefts result in a suspect being charged.'

The Met Police insists that thefts have fallen in recent months, with a spokesman adding: 'We are working hard to keep Londoners and visitors to our capital safe from thieves.'

Van Gastel's shouting outside Westminster Abbey alerted a security guard who came to inspect the commotion – but instead of making any attempt to apprehend the pickpockets, he told van Gastel to keep the noise down!

The intended victim, Wenhua Zhang, 54, who had travelled to London from Shanghai for a ten-day holiday, luckily had nothing taken from her bag.

We found her in the Abbey's gift shop with her daughter, Yixue Zhang, 28, a university lecturer. Both were completely oblivious to the fact they had been targeted.

When we showed Wenhua the footage she was shaken. 'Thank you so much,' she said in relief.

Van Gastel's skill, it becomes clear, is to work out quickly which members of a crowd are behaving suspiciously – usually long before they have committed a crime.

He explains the giveaway signs. 'The pickpockets look [at their targets] like this,' he says, demonstrating the look by staring at people's midriffs, the part of the body where pockets and handbags present tempting targets. 'They are looking: what can I get?'

Another telltale sign, he says, is a pashmina or oversized scarf. 'When they want to open a zip, they use their scarf to hide their hands. Nobody can see it – their hands are underneath.'

Umbrellas can also be used to obscure their intentions. 'With the umbrella, nobody can see what they are doing.'

Just ten minutes later, as we walk towards Buckingham Palace, it starts to rain and a number of tourists put up their brollies.

'Rain is good for us,' he says. He's right: he immediately spots three women, two of them with umbrellas. He had spied them earlier in the day on a street corner, counting out cash, and worked out they were street criminals.

'They often work in quite big teams,' Anne-Marie explains. 'And split up and share the cash – often throwing away the wallets.' Sometimes one crook bumps into you and, while you are distracted, the second dips into your bag.

We track the women, who are mingling with tourists by the railings of the Palace. Before I can work out what's going on, van Gastel starts shouting again: 'Pickpocket!'

The women, like the Westminster Abbey pair, nonchalantly walk away, not denying anything. Again, neither the police nor a Palace security guard is near enough to stop them.

I ask van Gastel if he is ever afraid his vigilante actions will end up with the criminals retaliating. He is, after all, a slight man, just 5ft 6in tall and 11st. 'I have no fear,' he says, matter-of-factly.

His very vocal tactics, combined with his YouTube videos – which have garnered 55 million views since he started publishing them in 2017 – have made him a minor celebrity in the Netherlands. As we walk near Buckingham Place, a Dutch tourist, Lucas Debruijn, 24, recognises him and tells me: 'He is an absolute hero in Holland! He is the guy who saves us from pickpockets.'

Not that it's made him rich. Anne-Marie says the money they earn from advertising on his YouTube channel is quite modest. 'It pays for our holidays,' she says.

We continue to patrol the area around Buckingham Palace and, before long, he spots two smartly dressed women who, he insists, are displaying all the telltale body language of pickpockets. Both look smart, one has a Goyard tote bag – a £3,000 accessory from a Paris designer – over her shoulder.

Surely, they can't be criminals if they are sporting such high-end accessories? 'Not at all,' says Anne-Marie. 'The pickpockets often wear Louis Vuitton or Prada. They are living the high life.'

Van Gastel dashes along the pavement to track them, with me and the Mail photographer following in his wake. Soon, one of them takes off her hat and places it over her hand as she moves behind a Chinese man wearing a backpack. The hat hides her hand movement – but she is clearly doing something with his backpack.

This is the trigger for van Gastel to start shouting: 'Pickpocket! Pickpocket! Why are you stealing? Go away!' Again, they do not protest; they just try to hide their faces from his camera and move away, while the Chinese man, seemingly unaware he had been a target, melts into the crowd.

The hollering, however, has worked for once. A constable patrolling 100 yards away, hears the commotion and calls for back-up. We only discover this a few minutes later when we see four officers questioning the women.

Van Gastel explains to the police he's the man responsible for the shouting and shows them the footage on his camcorder, which is enough to persuade them to arrest the two women.

They are bundled into separate vans and taken to Charing Cross police station. Van Gastel then spends an hour on a quiet street corner giving a full statement.

This rather brings a halt to our pickpocket-hunting, but it has been a remarkable afternoon. In just three hours he has halted three crimes.

However, I discover later the Met decided to take no further action against the two Croatian women due to lack of evidence – despite van Gasten's footage. Maybe it's not surprising that just 6 per cent of thefts from the person result in a charge.

Van Gastel and de Wit hit the streets the following day, too. He says that on Sunday he spotted a further 11 individuals and even managed to snatch a wallet off a crook and return it to a Chinese tourist, who had not been aware it had gone from their backpack.

'It's so great when we stop them. We stop a crime, we warn people,' he says. But for him the most important thing is for people to avoid the pickpockets in the first place. 'Place your bag on your front,' he suggests. 'And keep your wallet deep in your front pocket, not inside your bag.'

Are the streets really infested with gangs of pickpockets?

From the few hours we spent with van Gastel, sadly it would seem little has changed since Arthur Conan Doyle described the London of Sherlock Holmes as a 'great cesspool'.