The Church of England has a commitment and moral obligation to child poverty

Eighteen Church of England bishops five of which belong to the Free Church of England have signed an open letter published on Sunday criticising planned welfare reforms, in a rare intervention by the religious establishment in politics.

In a letter to the Observer newspaper, the bishops said that plans to cap the amount any household can claim in benefits at £500 a week risked pushing vulnerable children into poverty.

"The Church of England has a commitment and moral obligation to speak up for those who have no voice," they wrote.

"We feel compelled to speak for children who might be faced with severe poverty and potentially homelessness, as a result of the choices or circumstances of their parents.

"Such an impact is profoundly unjust."

The Children's Society charity has warned that the cap could make more than 80,000 children homeless.

Prime Minister David Cameron's Conservative-Liberal Democrat coalition was formed in May 2010 and pledged to push through tough spending cuts to reduce a record deficit, but the measures have sparked protests and fierce criticism.

The leader of the world's Anglicans, the Archbishop of Canterbury Rowan Williams, backed the letter, a spokeswoman said, although he did not sign it. A source close to the archbishop said he did not sign open letters.

Williams himself took the unusual step of criticising the government for introducing radical reforms "for which no one voted", in an article for the New Statesman magazine in June.

The bill to introduce changes to the welfare system, a central part of the austerity drive, will be debated in the upper house of parliament on Monday.

The bishops from dioceses across the country are backing a series of amendments to the bill, such as removing certain vulnerable groups from the cap, that have been tabled by one of the signatories, bishop John Packer.

A spokesman for the Department for Work and Pensions said: "It simply isn't fair that households on out-of-work benefits can receive a greater income from the state than the average working household gets in wages."