Ten thousand savvy savers have topped up the value of their state pension - one by as much as £148,922 - since April using a rare government offer.

A special window is open that lets you buy up to ten extra qualifying years towards your state pension, if you have gaps in your National Insurance (NI) record.

But hundreds of thousands of people who have yet to boost their retirement income have just six months left to do so.

The generous one-off offer has been dubbed 'one of the best investments you can make' by experts. They say the scheme means you could spend up to £8,000 and net a £55,000 boost to your income over a 20-year retirement.

In the most extreme example, one saver has managed to boost their state pension income by nearly £149,000. This would cost an estimated £15,500 today. 

The scheme means you could spend up to £8,240 and net a £88,306 boost to your income over a 20-year retirement assuming the state pension increases each year under the triple lock, calculations by wealth manager Quilter show.

Savers have made more than 10,000 payments worth £12.5million since April to boost their state pensions through the Government's new dedicated online service, figures released this week show.

The deadline for taking up the offer was supposed to be last April but was extended after Money Mail warned that many anxious savers were unable to top up their pensions due to administrative chaos at the Department for Work and Pensions.

The deadline was pushed back to April 5 next year, which means savers now have less than six months left to make use of the generous offer.

So how do you know if you are one of the hundreds of thousands that stands to gain? Here, we explain all you need to know to take advantage and how much it will cost you to boost your state pension in later life.

How much state pension should I get?

The state pension is currently paid from the age of 66 and the amount you get will depend on several factors. This includes when you reached retirement age, how many years you worked, your type of employment (self-employed or employed) and how much you earned.

There are two types of state pension. The 'basic' state pension is paid to those who reached pension age before April 6, 2016. The new 'flat-rate' state pension is paid to those who reach it after that date.

Anyone on the 'basic' state pension needed 30 years of NI contributions to qualify for £169.50 a week – or £8,814 a year. It came with various top-ups, including the second state pension and so-called 'contracted out' pensions.

Those on the 'new' state pension need a higher 35 years of NI contributions to get the full amount – £221.20 a week, or £11,502 a year.

Anyone who has not met these NI requirements will receive a smaller pension in retirement. It is therefore important to identify any shortfall if you were unaware they had occurred.

How can I boost my state pension?

The main way to increase the amount you receive in the state pension is by filling in any gaps you have in your NI contribution record.

You can pay to buy missing NI contributions for specific years during which you didn't make them, or didn't pay enough NI to clock up entitlement. This is unlike the NI contributions employed workers make each month – known as Class 3 contributions – which are paid as a percentage of your income.

Each week you want to make voluntary NI contributions for costs of £17.45 this tax year. So if you wanted to fill a whole year it would cost £909.95. At current rates, this boosts your pension by £329 a year. That's £6,573 over a 20-year retirement – without factoring in any increases in the state pension under the triple lock. This is the rate for Class 3 contributions, the most common type paid by workers.

Those who are self-employed typically pay at the Class 2 rate, which is much lower than for employees – currently £3.45 a week, or £179.90 a year. This means it costs much less to fill in gaps in a record but generates the same boost in the state pension.

If you have a partial year, for example where you worked for some of the year but didn't pay enough NI to make it a qualifying year, then the price of filling that year will often be a lot cheaper than a year which you missed entirely.

Most savers who boosted their state pension this year paid to top up one year's worth of NI contributions and paid an average of £1,193, according to official figures.

The single biggest increase that someone made to their state pension was by £107.44 a week. Money Mail analysis finds that this could add up to almost £149,000 for someone who lives until age 90 and the state pension rises by the minimum 2.5pc under the triple lock.

What is the deadline?

You can usually only fill gaps in your NI record that date back to the past six years. But a one-off window is currently open that allows you to fill any holes going back to 2006.

You have until April 5, 2025 to apply and make the payments for this. After that, the rules will revert to normal and you will only be able to cover the past six years.

But it is important not to leave it until the last minute if you plan to make any historical top-ups.

The Government has already pushed the deadline back to give those struggling time to get through to the Department for Work and Pensions the chance of topping up. This means they are unlikely to do so again.

Jon Greer, head of retirement policy at Quilter, says: 'Although April 2025 might seem far away, starting the process early is crucial if you want to avoid missing out on a substantial increase in your state pension. Previously, there were reports of jammed phone lines and slowed processing due to the high volume of last-minute applications. To avoid these issues, it's important to consider making an additional voluntary NI top-up as soon as possible and use the Check your State Pension forecast tool by logging into their online account or via the free and secure HMRC app.'

How do I know if I can use the offer?

To check your NI contributions record and how much you are on track to receive in the state pension contact the Future Pension Centre on 0800 731 0175 if you are below age 66, or the Pension Service for those above pension age on 0800 731 7898. You will be asked for your name, date of birth and NI number.

They can tell you which years you are eligible to make extra contributions for and if you will benefit. 

There are several reasons why you may be missing some years' worth of NI contributions. 

For example, you may have been employed part-time, earned less than the threshold for NI payments to be made or taken time out of work to look after children or elderly relatives.

You can also access your forecast online at gov.uk/check-state-pension.

If you transfer the money without checking first and it transpires you miscalculated, you risk losing that money for good as HM Revenue and Customs will not always reimburse you.

Once you know exactly which years of missing NI contributions you want to fill in, and how much you need to pay, you must make the payment to HMRC. First you will need a reference number.

Call the National Insurance Helpline on 0300 200 3500 and an adviser will confirm how much you would need to pay per year and give you an 18-digit payment reference. You can then make the payment via your online banking through the government website.

Select the 'pay by bank account' option. You'll then be directed to sign into your online or mobile banking account to approve your payment.

You can also send a cheque in the post to: HM Revenue and Customs, National Insurance Contributions and Employer Office, BX9 1AN.

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