One million couples have successfully claimed the Government's marriage tax allowance, but another 3.2 million eligible couples have so far failed to lodge a claim that could net them £100s, can reveal.

Marriage tax allowance provides a way for couples to transfer a proportion of their personal allowance (the amount you can earn tax-free each tax year) between them. To qualify, one of you must be a non-taxpayer and the other must be a basic-rate taxpayer.

The tax allowance for married couples started on 6 April 2015, and in its first year was worth £212. For the current tax year, starting in April 2016, it's worth £220 – plus, claim it now and it's backdated so you may get last year's AND this year's allowance, which works out at a cool £432.

The latest HM Revenue & Customs figures, shared exclusively with MSE, show that of the 4.2 million couples who are eligible to claim marriage tax allowance, a whopping 3.2 million have yet to make a claim – this is despite the fact the benefit has been highlighted in adverts in national papers and on the sides of buses.

Our marriage tax allowance guide takes a more in-depth look at how it works.

Could I be eligible?

Only people who fit the criteria below will be able to apply:

  • You're married or in a civil partnership (just living together doesn't count).
  • One of you needs to be a non-taxpayer, which usually just means earning less than the £11,000 personal allowance (£10,600 for 2015/16).
  • The other one needs to be a basic 20% rate taxpayer (couples with a higher- or additional-rate taxpayer aren't eligible for this allowance). This means you'd normally need to earn less than £43,000 (£42,385 for 2015/16).
  • Both of you must have been born on or after 6 April 1935 (if not, there's another tax perk).

How do we apply?

It's very simple, and only takes a few minutes – just use the application at HMRC. To do it you'll need both your national insurance numbers, and one of a range of different acceptable forms of ID for the non-taxpayer.

If there's a problem doing it via the web, just call 0300 200 3300 and do it over the phone.

It's worth noting you can only apply for those years in which you both meet or met the criteria above. So if you earned more than the £10,600 personal allowance in 2015/16, HMRC won't allow you to claim for it.

There is one very important point to make though...

It's the non-taxpayer who must apply to transfer their allowance.

If the taxpayer applies, you're doing it the wrong way round and it won't work.

You'll be notified immediately after applying if you're eligible for the allowance via email. If you were also eligible for the allowance in the 2015/16 tax year, you'll have to select this option as part of the application.

In most cases, the allowance will be given by adjusting the recipient partner's personal tax code. The partner who transferred their personal allowance will also receive a new tax code, if employed. If the recipient partner is in self-assessment, it will reduce their self-assessment bill.

Isn't this discriminatory against people who aren't married, or who earn more?

You could certainly make that argument. This is a Government policy to reward the institution of marriage (or civil partnership) – because it believes this formalisation of a relationship provides a more stable family.

The fact it's a transfer from the non-taxpayer to an earner also indicates a concept of rewarding the perhaps more old-fashioned 'one parent stays at home' set-up.