Martin Lewis issued an "urgent" six-week warning about the state pension top-up deadline that is fast approaching.
The Money Saving Expert shared the warning on the latest episode of his ITV show and said that many could miss out on thousands of pounds.
This could happen if you don't check your National Insurance years before April 5 this year - as this has an impact on how much state pension you could get.
To get the full state pension, you must have 35 years worth of National Insurance Contributions. You normally need ten years to get any state pension at all.
Currently, the full state pension is £185.15 per week, with the state pension age set at 66. However, Martin highlighted that many people are missing National Insurance years.
Martin said: "Maybe because you were caring for somebody or caring for a child or you had years abroad or you had a low income or you had a career break. Now, this is why it's so urgent."
The Money Saving Expert explained that April 6, 2016, was the day the Government introduced the new state pension and those who hit the pension age since then, have been put on the new state pension.
He added: "As part of that, transitional arrangements were put in place and those transitional arrangements end this tax year, tax year, that is April 5, and it's about six weeks away."
"Until then, you can plug any gaps back to 2006 in your National Insurance. But after that, you can only go back six tax years to 2017.
"So there are 11 years that you will lose on April 6 the ability to buy back. So I want you to check now whether you're missing any years. You can do that on the Government's website.
If you do have a shortfall, you are able to buy more years and Martin noted that if you have gaps from 2006 to 2016 you will need to decide what you want to do soon as the "window is closing".
The Money Saving Expert also noted that those who are younger have more times to plug the gaps by working and someone who is closer to the state pension age they are able to work out how much they need to top up.
A full National Insurance year usually costs £824 and adds up to £275 each year to your pre-tax state pension.
Martin added: "A full voluntary national insurance should cost around £800 and adds £275 years to your state pensions.
"At 66 and man will typically live 19 more years, so with £800 pounds, we're getting around £5,300 extra at age 66.
"Typically, a woman will typically live 21 more years so it would get you an extra £5,800 quid."
The Money Saving Expert then gave viewers some "baboon warnings" as there are a few "big butts" with topping up your National Insurance Credits.
He said: "If you do this, it might diminish the amount of pension credit you're entitled to. If you do this by getting more it might push you into a higher tax bracket which means the returns may not be what you calculated.
"This is not science to an extent, it is a little bit of art, but for the vast majority of people out there, if you have gaps in those years, especially if you're nearing retirement age, it will be worth doing them."
