The Archbishop of Canterbury and Tory former Cabinet ministers have helped inflict the heaviest defeats Rishi Sunak has suffered in the Lords over his proposed Rwanda asylum law.

The string of Government setbacks, most passed by unusually large margins of more than 100 votes, set the stage for a Westminster showdown over the legislation that aims to clear the way to send asylum seekers who cross the Channel in small boats on a one-way flight to Kigali.

The Prime Minister has previously warned the unelected chamber against frustrating “the will of the people” by hampering the passage of his Safety of Rwanda (Asylum and Immigration) Bill, which has already been approved by MPs.

Ahead of the next election, Mr Sunak has made “stopping the boats” a key pledge of his leadership.

The draft legislation and a treaty with Rwanda are intended to prevent further legal challenges to the stalled deportation scheme after the Supreme Court ruled the plan was unlawful.

As well as compelling judges to regard the east African country as safe, it would also give ministers the power to ignore emergency injunctions.

It has been warned that the legislation is “fundamentally incompatible” with the UK’s human rights obligations and would flout international law.

But Downing Street has said the Government remains committed to sending flights to Rwanda “in the spring”.

In all, peers backed five changes to the Bill, including ensuring it complied with the rule of law and that Parliament cannot declare Rwanda to be safe until the treaty with its promised safeguards, is fully implemented.

The Lords also supported a move that would allow the presumption the country is a secure haven to be challenged in the courts.

Among those to vote against the Government were the Most Rev Justin Welby, and Conservative grandees Lord Clarke of Nottingham, Lord Deben and Viscount Hailsham, who have all previously held Cabinet positions.

The size of the defeats raise the prospect of an extended tussle between the Commons and Lords during “ping-pong”, where legislation is batted between the two Houses until agreement is reached.

Labour former home secretary Lord Blunkett said: “When circumstances change most people change their minds.

“But if minds are not allowed to be changed when circumstances change, then we are all extremely foolish.”

Leading lawyer and independent crossbencher, Lord Anderson of Ipswich, said the provision in the Bill requiring Rwanda to be treated as safe “takes us for fools”.

Proposing an amendment that would allow the presumption to be challenged in the courts, he added: “If Rwanda is safe as the Government would have us declare, it has nothing to fear from such scrutiny.

“Yet we are invited to adopt a fiction, to wrap it in the cloak of parliamentary sovereignty and to grant it permanent immunity from challenge.

“To tell an untruth and call it truth.

“Why would we go along with that?”

Mr Welby said international human rights law had developed following the horrors committed by Nazi Germany, to act as a “fallback” and “stop” on governments.

He added: “We are not in any situation remotely like that let’s be clear.

“The Government is not doing something on the scale of what we saw at that stage, but the Government is challenging the right of international law to constrain our actions.”

Urging peers to back the Bill, Tory former Cabinet minister Lord Lilley: “It is very important that we not only use common sense but be merciful, because the longer we delay the more people will come across the Channel, and the more will die.”

Former Conservative Party leader Michael Howard told peers the Supreme Court had been “trespassing” on the Government’s responsibilities in its Rwanda ruling.

Lord Howard of Lympne added: “Accountability is at the heart of democracy.

“That is why the Government is fully entitled to bring forward this Bill and why much of the criticism which is directed at it for doing so is… fundamentally misconceived.”

However, Tory former chancellor Ken Clarke said he hoped there would be a legal challenge to the Bill if it was passed.

Lord Clarke said: “I cannot recall a precedent in my time where a Government of any complexion have produced a Bill which asserts a matter of fact – facts to be fact.”

Later, responding to concerns about mental health support asylum seekers would receive in Rwanda, Home Office minister Lord Sharpe of Epsom said: “It would be far in the best interests, mental health interests of those seeking asylum, and who are victims, to seek asylum in the first safe country they came to.”

The Government faces the threat of further defeats on Wednesday when the Bill is again before the Lords.