Just two days remain for people to take a "once in a lifetime" opportunity to buy a bigger state pension, experts have warned. People build up an entitlement to the state pension by paying national insurance (NI) contributions or getting credits - for example by getting benefits - but if you miss years during your working life, it can reduce the size of the state pension you get. Until this Saturday 5 April, you can top up any missing contributions over the past 19 years, but after this date it will only be possible to go back six years. Failing to top up gaps in your state pension record this week could risk you losing as much as £85,000 in state pension income over a 20-year retirement, according to ex-pensions minister Sir Steve Webb. Sir Steve, now a partner at LCP, said it was "critically important" for people to focus on this Saturday's deadline. Anyone retiring now with 35 years of national insurance contributions or credits gets the full new state pension and those with a lower number of years get a smaller amount. Currently, the new state pension is worth £221.20 per week, but is rising by 4.1 per cent to £230.25 per week from 6 April under the triple-lock pledge. Many pensioners say this amount isn't enough to live on without other streams of income like private pensions. Here, The i Paper explains how you can top up your state pension record before it disappears forever. Until this Saturday, you can top up any missing contributions going back to the 2006/07 tax year. After this date, the window will close, and it will only be possible to go back six years. Despite the rise in the state pension next week, figures from pension firm Royal London show that only around half of people on the new state pension actually receive the full amount. This is why experts, including former pensions minister Ros Altmann, are urging people who are heading for a smaller state pension to top up their incomplete years. She said: "I would urge anyone who does not have a full NI record to look into buying added years or claiming credits for past missing or incomplete years. "This may cost nothing but give you a lot more pension in retirement and even paying for these years will normally be a good deal as long as you live for about three years beyond state pension age and have checked that your extra contributions will give you a better pension without losing other benefits." For many, filling NI gaps could mean a bigger state pension in retirement. But deciding whether to top-up - and which years to pay for - can be tricky. Costs vary depending on past contributions, and not all missing years will actually boost someone's pension. Craig Rickman, personal finance expert at interactive investor, said: "If you have gaps in your NI record stretching back to 2006/07, plugging them with voluntary contributions is strongly worth considering. "This won't be the right course of action for everyone, but for many it could make a marked difference to your future financial security." Topping up your NI record by just a single year's qualifying NI contributions costs around £800 to £900 but will boost your retirement income by over £300 annually, which could run into several thousands of pounds if you live a long and healthy life. What's more, future state pension increases are protected from rising costs thanks to the triple-lock mechanism, Mr Rickman said. He added: "To put the value of the state pension into context, buying a guaranteed, inflation-proofed income equivalent to the full amount (just under £12,000 per year from next week) would require personal savings in the region of a hefty £250,000. "But before you dip into your savings and fill any missing years, there are a few things to consider." The first thing is your life expectancy. If it's particularly short, you may not live long enough to recover your contribution. Secondly, if you've taken time out of the workplace, for instance, to raise young children, you may be entitled to NI credits. You may be able to plug your gaps for free with these instead of paying. Thirdly, if you've got many working years ahead of you, top-ups might not be needed, because 35 years of qualifying NI contributions will secure the maximum state pension, which you may achieve by the time you reach state pension age without buying a top-up. For example, if you contracted out of a Serps additional state pension scheme, there will be a point where no extra contributions can be made, as you'll already have reached the 'flat rate' of pay available. The quickest and easiest way for people to check a state pension forecast and find out if they can benefit from paying to fill NI gaps is by checking their NI record on GOV.UK or in the HMRC app.