British families with stay-at-home mothers pay more taxes than the rest of the Western world, a study reveals.
The burden they face is now a third higher than the average among the richest countries.
Charity CARE said households where one parent works and the other looks after children have been heavily penalised since David Cameron first entered Downing Street.
One-earner married couples with two children in the UK on the Organisation for Economic Co-operation and Development average wage of £35,448 are liable for 35 per cent more than the OECD typical tax bill.
Single-earner married parents in Canada, Ireland and Australia are next on the list.
The same families in Chile, the Czech Republic and Mexico pay the least tax.
The Marriage Allowance, introduced by Chancellor George Osborne last April, allows the non-working spouse or civil partner to transfer up to £212 of their £10,600 tax-free Personal Allowance to their partner.
Now the social policy charity Christian Action Research and Education (CARE) is calling for an increase in that amount for married couples in the emergency July Budget.
CARE’s CEO Nola Leach said: “It’s staggering that the UK’s one-earner married couples are still being so heavily penalised by the tax system.
“Given that marriage is by every benchmark the most stable form of relationship it does not make sense we should make the option of marrying fiscally less accessible in this country.
“Stay-at-home parents are making an important investment in their children and yet they end up being discriminated against by our current tax system, which surely a One Nation Prime Minister cannot be happy with.”
The analysis also said that because of tax credits – introduced by Gordon Brown more than a decade ago but with a future thrown into doubt by Mr Cameron – the effective marginal tax rate is 73 per cent for those on less than three quarters of the £35,448 pay level.
This means that for every extra pound they earn, 73p is lost in tax or benefit they no longer qualify for.
The report said that figures calculated by the Institute for Fiscal Studies think-tank show an average one-income family lost nearly £2,000 between 2010 and 2015.