Money expert Martin Lewis is urging millions of Brits to claim cash they are missing out on thanks to a little-known benefit which can be worth as much as £1,250 if you backdate it.

Speaking on the latest instalment of The Martin Lewis Podcast, Money Saving Expert founder Martin Lewis urged the 2.1m people across the UK who are owed as much as £250 a year to make sure they get their claim in with HMRC.

Martin explained to his co-host Adrian Chiles how Marriage Allowance could net you £250 a year and can be backdated for four years on top of your current year too.

He said: “So there are 2.1 million eligible married couples who are not claiming this, and this is a big tax break.

“It’s deliberate social engineering in the tax system to reward marriage or civil partnerships, if you are cohabiting it does not count.

“If you are in that case, then the tax break works like this: [co host] Adrian and I are married, you’re a non-taxpayer for the sake of argument in this case and you earn less than £12,570 in this scenario, and you’re married to someone whose highest rate of tax they pay is a 20 percent tax rate, you are allowed to transfer 10 percent of your Personal Allowance, the amount you can earn each year tax-free, which is £1,260 odd quid, and you can give that to me.

“So your unused allowance is coming to me so I now have £1,260 worth of income that is untaxed which was being taxed at 20 percent, a net gain of around £250 odd quid.

“But if we’d been married for those past four or five years and this had always been our scenario, we can back-claim four years.

“That’s around a grand if you’re claiming four years.”

Martin added that you’ll still get another £250 back in your tax income this year on top of the backdating, for a total of £1,250.

He added: “Once you’ve done it, as long as you’re still eligible they’ll keep doing it every year.

“It’s an absolute no-brainer but 2.1m eligible couples are still not claiming.

“But you have to claim it - I as the taxpayer cannot claim it, you have to apply to give me your 10 percent tax allowance - I can’t take 10 percent of your allowance.”