Millions of children around the world have not been vaccinated against measles amid a rise in online 'antivaxx' scare stories.
A surge in measles outbreaks around the world is being fuelled by plummeting vaccination rates, the charity Unicef has warned.
Around 527,000 children in the UK did not receive the potentially life-saving jab between 2010 and 2017, with 'fake news' spread on social media partly to blame.
The UK comes third in a global ranking showing the number of unvaccinated children in high-income countries.
The US topped the charts, with 2,593,000 youngsters not receiving their first measles dose over the eight-year-period.
Around 527,000 children in the UK did not receive the potentially life-saving jab between 2010 and 2017, with 'fake news' spread on social media partly to blame. The UK comes third in a global ranking showing the number of unvaccinated children in high-income countries
Simon Stevens, the head of the NHS in England, said: 'With measles cases almost quadrupling in England in just one year, it is grossly irresponsible for anybody to spread scare stories about vaccines.
'And social media firms should have a zero tolerance approach towards this dangerous content.'
Mr Stevens described vaccine rejection as a 'growing public health timebomb', warning parents it is essential their children are vaccinated against measles.
An estimated 169million children around the world missed out on the first dose of the measles vaccine between 2010 and 2017 - an average of 21.1million a year.
The antivaxx movement has seen the US hit with more measles cases in the first three months of this year than in the whole of 2018.
Some 626 cases have been recorded in the US in the last four months alone - nearly eclipsing the 667 recorded in all of 2014.
France came second in the Unicef rankings, with 608,000 unvaccinated children, followed by the UK.
Experts believe 95 per cent of children need to be vaccinated to protect the wider population through 'herd immunity'.
A surge in measles outbreaks around the world is being fuelled by plummeting vaccination rates, the charity Unicef has warned
Measles is a highly contagious viral infection that spreads easily from an infected person by coughing, sneezing or even just breathing.
Symptoms develop between six and 19 days after infection, and include a runny nose, cough, sore eyes, a fever and a rash.
The rash appears as red and blotchy marks on the hairline that travel down over several days, turning brown and eventually fading.
Some children complain of disliking bright lights or develop white spots with red backgrounds on their tongue.
In one in 15 cases, measles can cause life-threatening complications including pneumonia, convulsions and encephalitis.
Dr Ava Easton, chief executive of the Encephalitis Society told MailOnline: 'Measles can be very serious.
'[It] can cause encephalitis which is inflammation of the brain.
'Encephalitis can result in death or disability.'
Treatment focuses on staying hydrated, resting and taking painkillers, if necessary.
Measles can be prevented by receiving two vaccinations, the first at 13 months old and the second at three years and four months to five years old.
Source: Great Ormond Street Hospital
But in the UK uptake dropped to 87 per cent last year.
