MARTIN Lewis has warned of an "absolutely crucial" check parents must make that could see them get £48,000 for free.
The Money Saving Expert shared the advice on his show this evening and said about 200,000 families are currently missing out on the cash.
He explained this could be happening as the wrong parent is collecting National Insurance (NI) credits after they have kids.
Martin encouraged Brits to check who in the house is - or was - getting their credits as it could be a costly mistake.
He said on tonight's The Martin Lewis Money Show: "There are 200,000 of you out there who are missing out on potentially £10,000s of state pensions because the wrong partner in your relationship is getting the National Insurance credits.
"Now you build up National Insurance years that gives you how much of the full state pension you get in the end."
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NI credits are a way of maintaining your NI record when you are not making NI contributions.
They help to build up qualifying years over time, which you can use to make you eligible for basic state pension and other benefits.
You need 35 years of NI credits to get the maximum state pension under new rules, which is curranty worth £179.60 a week, and a minimum of 10 years to get anything at all.
Martin tonight explained that the non-working partner in any relationship should be collecting the credits.
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One thankful fan wrote into the show and said her family had managed to top up her husband's state pension.
The woman explained it came after she applied to transfer eight years' worth of credits, as she had been claiming the child benefit while working - rather than him.
Martin then used the families' example to explain how people could get their hands on the money.
He said: "Eight years, each boosts your pension by £300 a year.
"So that's a boost on the final state pension of £2,400 a year, if you live 20 years that's £48,000 extra state pension."
It comes as Martin revealed two easy checks that anyone under 73 should do to see if they are eligible for free state pension cash.
The consumer expert shared the advice in the latest MoneySavingExpert (MSE) newsletter and you could be set to gain tens of thousands of pounds.
The new state pension was introduced in 2016 for men born after April 5, 1951 - up to the age of 73 - and women after April 5, 1953 - up to 71.
You need at least ten years of qualifying NI contributions to get any state pension payments.
The first check he suggests making is whether you have missing NI years.
You might have holes in your record for various reasons, like if you took time out to raise children, but luckily there is a way to fix it.
If you don't fill in the gaps you could end up missing out on the full state pension when you retire, which at the moment is worth £203.85 a week or £10,608 a year.
But if you do have holes, you can pay for the gaps to be filled.
Currently you can backfill holes from 2006 to 2016 and you have until April 5, 2025 to do this.
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After this date, you will only be able to backdate payments by up to six years.
This scheme only applies to people who reached (or will reach) state pension age after April 5, 2016.