Robert Jenrick has resigned as immigration minister, saying the government’s emergency Rwanda legislation “does not go far enough”.
He said “stronger protections” were needed to end “the merry-go-round of legal challenges which risk paralysing the scheme”.
The government said the bill, unveiled earlier, made clear in UK law Rwanda was a safe country for asylum seekers.
But it stops short of what some on the Tory right were demanding.
In his resignation letter to Rishi Sunak, Mr Jenrick said the prime minister had “moved towards my position” on the emergency legislation.
“Nevertheless, I am unable to take the currently proposed legislation through the Commons as I do not believe it provides us with the best possible chance of success.”
Mr Jenrick added that the bill was “a triumph of hope over experience”.
In response, the prime minister described Mr Jenrick’s resignation as “disappointing” and “based on a fundamental misunderstanding of the situation”.
“If we were to oust the courts entirely, we would collapse the entire scheme,” Mr Sunak said.
“The Rwandan government have been clear that they would not accept the UK basing this scheme on legislation that could be considered in breach of our international law obligations.”
The plans to send some asylum seekers to Rwanda aim to deter people from crossing the English Channel in small boats.
But the scheme has been repeatedly delayed by legal challenges and no asylum seekers have been sent to the east African country from the UK so far.
Mr Jenrick, who had supported Mr Sunak’s leadership campaign, said the emergency legislation was the “last opportunity” to prove the government would do “whatever it takes” to stop small boat crossings.
Losing a minister who was once a key ally is a blow for Mr Sunak, in a week when the government had been trying to get on the front foot on migration.
Labour’s shadow home secretary Yvette Cooper said: “It is a sign of the total chaos in the Tory party and the complete collapse of Rishi Sunak’s leadership that even while he is sitting in the Commons for the announcement of his new Rwanda plan, his own immigration minister is resigning because he doesn’t think it will work.”
Senior figures are musing privately that they would not be surprised if Mr Sunak ended up facing a confidence vote from his own MPs.
Although it may not come to that, it is a measure of the bleak mood among many Conservatives.