Former immigration minister Robert Jenrick has said Rishi Sunak is making “a political choice” to revive the Rwanda asylum scheme in a way that is unlikely to work.
In his first interview since quitting his post, Mr Jenrick told the BBC the draft law was “weak” and would be “bogged down” by legal challenges.
But he said the PM had made a decision to introduce it anyway.
He added that he would not support the bill at a crucial vote on Tuesday.
The government has introduced the legislation to revive its scheme to send people seeking UK asylum to the east African country, after the policy was ruled unlawful by the Supreme Court last month.
The legislation seeks to declare in UK law that Rwanda is a safe country to send refugees to – thereby stopping flights being grounded on legal grounds.
But, speaking to BBC One’s Sunday with Laura Kuenssberg, Mr Jenrick said the legislation in its current form would still allow a “range of legal claims that will bog down our scheme”.
“I think that a political choice has been made to bring forward a bill which doesn’t do the job,” he added.
Although he conceded the Rwanda plan could eventually lead to some “symbolic” flights taking off, there would not be enough deportations to deter people from crossing the English Channel in small boats to make an asylum claim.
The bill to revive the scheme has divided opinions among Conservative MPs, whose support Mr Sunak will need to ensure the bill passes its first parliamentary stage next week.
The prime minister insisted in a news conference last week that the legislation was the “toughest immigration law ever”, and only allowed for individual legal challenges if the small boat arrival can prove “with credible and compelling evidence” they have a “real and imminent risk of serious and irreversible harm”.
Some liberal Tories have argued it goes too far in trying to bypass the courts, and puts the UK at risk of breaching its international legal obligations towards refugees.
But others on the right of the party say it does not go far enough, and will continue to be derailed by legal challenges.
Mr Jenrick added that he, along with like-minded Conservative MPs, hoped to persuade ministers “that there is a better way” to design the new law, adding he wanted the bill to work and create a “powerful deterrent”.
Speaking on the same programme, Housing and Communities Secretary Michael Gove defended the government’s legislation, saying it was a “robust bill that is legally sound”.
He also said it was “very clear” that the number of legal challenges would be “vanishingly small”.