Tony Blair is facing his biggest confrontation with the Church of England since coming to power over the Government's plans for reform of the House of Lords.
Senior bishops are furious that they are not represented on the joint committee being set up to draw up proposals for a new second chamber.
The Archbishop of Canterbury, Dr George Carey, has complained to the Prime Minister that the established Church is being sidelined in discussions about the future of the Upper Chamber.
Senior bishops, led by Richard Harries, the Bishop of Oxford, are planning to raise concerns about the make-up of the joint committee when the matter is debated in the Lords on Thursday. They are considering forcing a vote in an attempt to gain representation on the committee.
The churchmen are concerned that excluding the bishops is the first step towards removing them from the Lords altogether and, eventually, disestablishing the Church of England. The bishops' presence in the Lords is one of the main symbols of the constitutional role of the Church.
Labour peers are angry at the attempt by the bishops to fight their way on to the committee. "If they're going to act like politicians then they should expect to be treated like politicians," said one.