Jack Straw has warned Rishi Sunak he will be left “embarrassed” by the latest plan to tackle small boats crossing the Channel.
Straw said it was a “desperate measure” that would not work and ultimately leave the prime minister humiliated.
The new legislation will place a duty on the home secretary to remove anyone who enters the UK by an illegal route.
It will also prevent those arriving illegally from claiming asylum or returning to the UK in future.
Straw told Sky News: “They keep coming up with these desperate measures.
“This latest measure - which is to try and override all these international obligations and turn ourselves into a kind of pariah amongst western European states - is not going to work anyway.
“I promise you, in a year to 18 months time when there’s a general election Mr Sunak will be very, very embarrassed about the fact that the numbers may have come down a bit, but they have not stopped.”
Asked how banning an asylum seeker for life could work, Straw replied: “I don’t know. This sounds like stuff written on the back of an envelope for the benefit of - red meat if you like - being chucked at the right wing of the Tory party, without really thinking it through.”
Home secretary Suella Braverman said the bill would push “the boundaries of international law”, telling the Express this was needed to “solve this crisis”.
Labour leader Sir Keir Starmer has described the plans as “unworkable” and the Lib Dems slammed them as “immoral, ineffective and costly”.
The plans have been likened to the failed Rwanda scheme - a controversial policy unveiled in April last year, with former prime minister Boris Johnson saying action was needed to stop “vile people smugglers”.
However, no-one has been sent to the African country and thousands have continued to cross the world’s busiest shipping lane in precarious dinghies.
However, even Tory ministers have admitted the policy would only have “marginal benefit” in stopping asylum seekers crossing the Channel.
The latest Home Office figures show 2,950 migrants have crossed the Channel already this year.
Writing in The Sun, Sunak said the plans were “fair for those at home and those who have a legitimate claim to asylum”.
A Home Office spokesperson said: “The home secretary has been clear that if you arrive in the UK illegally, you should not be allowed to stay.
“We will shortly introduce legislation which will ensure that people arriving in the UK illegally are detained and promptly returned to their home country or a safe third country.
“Our work with France is also vital to tackling the unacceptable rise in dangerous Channel crossings. We share a determination to tackle this issue together, head-on, to stop the boats.”