Christian Action Research and Education (CARE) has warned that families in the UK are facing a "crippling" tax burden.
In its 'Taxation of Families' report, published today, the advocacy group said that under the current system, families in the UK are faring much worse than their counterparts in other developed countries.
In particular, it is concerned for households with a single earner, which it says are facing a "triple whammy" under the current tax system.
For those earning the OECD average wage (£39,328 for the UK), the report found that the overall tax burden in the UK is 28 per cent greater than the OECD average both for single parents with two children and one-earner married couples with two children.
In the UK, a married couple with two children but only one earner can expect to pay 39 per cent more than the equivalent French family, three times as much as a US family and more than 10 times as much as a German family, CARE said.
The organisation went on to warn that aspiration was being "chocked" by an "absurdly high" effective marginal tax rate (EMTR) of 73%, which allows families to keep only 27 pence of every additional pound earned. It amounts to the highest EMTR on families in the developed world and is far above the OECD average of 34%.
Those on housing benefit in the UK fare even worse, with an EMTR of 90.6%.
CARE said that the income tax burden was putting families in the UK at a "strong disadvantage".
With the new Budget coming out next week, CARE is calling for a review of the entire income tax system, paying special attention to families, and an enhanced marriage tax allowance.
CARE's Chief Executive Nola Leach, said: "The UK is a cold place for families, with one-earner married couples with children facing a triple whammy, which is crushing aspiration and social mobility.
"The fault does not lie in an individualised tax system, but rather in the fact our tax system is hyper-individualised, with little or no recognition of family responsibilities.
"In a country that values aspiration and with a Government committed to social mobility, this cannot be right.
"For one-earner married couples with two children on the OECD average wage for the UK, the overall tax burden is suffocating. The income tax burden is hardly better especially when compared with France, Germany and the United States.
"To further add to the pressure modest income families are facing, the UK's eye-wateringly high effective marginal tax rates are crushing aspiration.
"If the Prime Minister is serious about levelling up across the country, he cannot ignore the way the tax system discriminates so obviously against those with family responsibilities."
