Families in the UK where one parent stays at home to look after the children pay more than a third extra in tax in Britain than similar families in other Western countries, according to a new report.

British single-earner families with two children on £33,745 a year were paying 39 per cent more in tax than comparable families from Organisation for Economic Co-operation and Development (OECD) countries, according to the study.

In Ireland, one-income families pay far more tax than two-earner families on comparable incomes mainly because of tax individualisation.

The report warned that disparity in the UK would worsen to 50 per cent by 2012-13. It said that part of the blame was due to the Coalition’s controversial decision to axe child benefit for families where one parent earns more than £42,000.

Researchers for CARE, a Christian social policy charity, said the tax system was “unfriendly to families” and was going to get worse.

They said that “those in the middle who are not rich are shouldering a heavy burden”, despite Prime Minister David Cameron’s election pledge to make Britain the most family-friendly country in Europe.

The charity recommends introducing transferable allowances for married couples, which were outlined in the Coalition agreement. The report, “The Taxation of Families 2009-10”, is a detailed assessment of the impact of the tax system on families compared with 33 other countries.

About 2.4 million children in Britain live in households where one parent is in full-time work and one is not working.

During the past few years, the report found, the tax burden had shifted markedly from single people without dependants to families. While the tax burden on most families and individuals was “not out of line with that in other countries”, it found that “this is not the case with one-earner married couples with children”.

From 2012-13, one-earner couples with two children who earn slightly more than £42,000 would be hit with an effective 25 per cent increase in their tax burden, compared with single people with no dependants on the same wage. The withdrawal of child benefit would have little effect on the richest households in the UK, but would impact on those around the middle and the upper-lower half of income distribution, CARE said.

Nola Leach, the chief executive of CARE, said: “The treatment of married couples on modest and average incomes in the tax system remains unfair and out of line with the rest of the OECD. This failing is damaging family life.”

Meanwhile, the UK Government is set to propose changes to the pension regime which will remove the penalty for working mothers who take time out to look after their children.

The Work and Pensions Secretary Iain Duncan Smith is set to promise to get rid of a range of complex rules and “fundamentally simplify” the basic state pension.

It is estimated that many women who take time out from work for family reasons are left up to £40 a week worse off by rules that base pension payments on National Insurance contributions.

Last month, Department of Work and Pension figures suggested that leaves women an average of £2,000 a year worse off than men in retirement.