The archbishops of Canterbury and York have strongly rebuked the government over the behaviour of Western security forces in Iraq.

In a joint attack, Dr Rowan Williams and Dr David Hope accused the prime minister of "double standards" and warned that the credibility of his government is at risk over the treatment of Iraqi detainees.

In a strongly worded letter, written on behalf of all Church of England bishops, they said: "It is clear that the apparent breach of international law in relation to the treatment of Iraqi detainees has been deeply damaging.

"The appearance of double standards inevitably diminishes the credibility of Western governments with the people of Iraq and of the Islamic world more generally.

"More fundamentally still, there is a wider risk to our own integrity if we no longer experience a sense of moral shock at the enormity of what appears to have been inflicted on those who were in the custody of Western security forces."

The letter, written with the unanimous support of all the Church's 120 diocesan, suffragan and assistant bishops, was sent to the prime minister on Friday.

Downing Street confirmed last night that it had been received. An official said: "The archbishops are entitled to their views and the prime minister will reply in due course."

It also emerged yesterday that the prime minister was aware of inappropriate treatment of prisoners as early as May 23.