The Government could be plunged into a “constitutional crisis” if members of the House of Lords reject plans for gay marriage this week, a senior peer has warned.

Opponents of the Same Sex Couples Bill will force peers to vote on Tuesday on a “fatal motion” to kill off the Bill before it is even considered in detail by the Lords. Should the attempt succeed, David Cameron will be placed in the contentious position of forcing through the Bill with a rare use of the Parliament Act or abandoning the current legislation altogether.

In an interview with The Independent, the Labour peer Lord Alli, a staunch supporter of the Bill, warned that tomorrow’s vote was “too close to call” and that it could trigger a crisis for the Government should the Bill fail.

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“If they win the vote then the Bill doesn’t come to us and the Commons can’t do anything about it,” he said. “The Government could use the Parliament Act but the argument against it is that the issue was not a manifesto commitment. In my view they could legitimately do it because it was a free vote – but it’s a constitutional crisis,” he added.

Lord Alli also revealed that the new Archbishop of Canterbury, Justin Welby, had been keen to allow a contrary view to be heard from within the Church, despite his hardline public stance in opposition to gay marriage. He is understood personally to have sanctioned a public letter to Lord Alli, written by the Bishop of Salisbury, which argued that gay marriage “now needs recognition in law”.

Lord Alli told The Independent: “I said [to Archbishop Welby] that I knew there were people in the Church like the Bishop of Salisbury who are supportive of gay marriage.

“I asked him: ‘If I went to see him and asked him to do a piece, would he have your blessing?’

“He said, ‘Absolutely. And that goes for any bishop.’”

All 26 bishops who sit in the House of Lords will be given a free vote on the issue in Tuesday's vote with the vast majority expected to oppose the Bill. However, three could vote in favour or abstain.

Archbishop Welby is also expected to press the Government to accept amendments that would ensure teachers would not have to promote equal marriage in the classroom.

Lord Dear, who is leading the opposition to a change in the law, said he also expected the vote to be tight and predicted that even more Lords would oppose it at a third reading.

However he said Mr Cameron would have no justification in using the Parliament Act should it be defeated on a second reading.

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“This Bill was not in the Conservative manifesto, has not been subject to a Royal Commission, Green Paper or White Paper or any of the normal procedures and safeguards you would expect for legislation of this significance.

“We are saying that the Government should go away and consider far more carefully than it has done the unforeseen consequences of such far-reaching social legislation.”

The former Tory cabinet minister Lord Mawhinney said the advice the Prime Minister was getting was not “politically astute”.

“At a time when the Government should be focusing all its attention on the economy, which is what will decide the next election, we have this distraction following Lords reform distraction, following Alternative Vote distraction.

“This is not a high priority for the people of the country.”

Last resort: the parliament act

The 1949 Parliament Act has only been used four times since it was amended more than 60 years ago. It allows a law to be passed without the approval of the House of Lords and is sometimes billed as Parliament’s “nuclear deterrent”, a term that highlights the magnitude of using the contentious legislation.

The amended Act of 1949 reduces the time peers can delay a Bill from two years over three parliamentary sessions to one year over two sessions.

It was last used in 2004 by Tony Blair’s Labour government to push through its fox-hunting ban, which faced fierce opposition in the Lords – and had prompted demonstrations by people opposed to the ban. The legality of its use was unsuccessfully challenged by pro-hunting groups who took the case to the Administrative Court and Court of Appeal in 2005.

In 2000, after months of debate, the Parliament Act allowed the same government to lower the age of consent for homosexual sex from 18 to 16. The use of the Act was described as a “draconian device” intended to derail public opinion by Christian campaign groups.

It was used on two previous occasions: in 1999 to change the way Members of the European Parliament were elected from the first-past-the-post system to a form of proportional representation; and in 1991 for the War Crimes Act which gave UK courts the right to try suspected Nazi war criminals.

Alex Delmar-Morgan