Several bishops and dozens of rabbis have led calls for the most vulnerable children in Europe to be fast-tracked into the UK in time for the school term in September.

A letter in the Times signed by faith leaders including the former Archbishop of Canterbury Lord Williams offered to help as they said more than 300 children who are most at risk should be "brought to Britain by the start of the next school year".

The charity Unicef and Yvette Cooper, chair of Labour's refugee taskforce, has made similar calls. Cooper said it was the "minimum we should be doing and I urge you to listen to the call from faith leaders and make this commitment today".

The joint calls come after Number 10 said on Monday that Britain would not be able to accept any refugee children from Europe for another seven months. "It is a new scheme so we need to work with councils to get it up and running and that is why we expect we will have the first children arriving by the end of this year," said the Prime Minister's spokesman.

The news prompted outrage from a number of Conservative MPs, including many Christians, who had threatened to rebel if the government did not change its opposition to taking unaccompanied children from Europe.

David Burrowes, Tory MP and former chair of the Conservative Christian Fellowship, said: "The PM's decision last week was a response to an urgent humanitarian need and demands and public expects an urgent response.

"We were able to relocate 1000 vulnerable refugees from Syria in three months so it should not take seven months before we relocate vulnerable children from Europe."

Last week Cameron announced a U-turn and said Britain would take unaccompanied child refugees from camps in Europe. However he did not say how many would be accepted and a time-frame was not clear until the statement from Number 10 on Monday.

The charity Citizens UK has said 157 children from the Jungle camp in Calais have family in the UK. In their letter the faith leaders say "all the children in Calais with valid legal claims, and the first 300 identified as most at risk in Greece and Italy" should be fast-tracked to the UK.

The letter celebrates the government's "bold and decent decision" to reverse its policy but adds, "we must not forget the urgency of the situation".

The letter is signed by the bishops of Durham, Manchester, Barking, Croydon and Stepney as well as the former Archbishop of Canterbury and Territorial Commander of the Salvation Army in the UK and Ireland. Dozens of the UK's most senior Rabbis also signed the letter.