The EU withdrawal bill – necessary to prepare UK laws for exit day next March – may not complete its passage until after the summer recess, No 10 said for the first time.
It is certain to add to criticism that the government is in danger of losing control of the Brexit process, as the cabinet continues to argue over the terms of exit.
Andrea Leadsom, the Commons leader, refused to give a date for the bill’s return, saying more time was needed to “take into account the views expressed by the other place”.
Asked if the prime minister still expected it to pass into law before MPs leave Westminster, in late July, a No 10 spokeswoman declined to say she did.
“Given how important it is, we are just taking some time to make sure that we consider all the amendments thoroughly,” she said.
The legislation was ripped apart during its during its 20-day passage through the Lords, including in the other key areas of the Irish border and a decisive say for parliament if the government’s deal is rejected.
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Tory rebels are confident they can repeat some of those defeats in the Commons – prompting a fearful Downing Street to put the bill on hold.
Jenny Chapman, Labour’s shadow Brexit minister, said: “Theresa May’s flagship Brexit legislation has gone from being the Great Repeal Bill to the Great Delayed Bill.
“It is frankly embarrassing that nearly two years since the referendum ministers have failed to pass a single piece of Brexit legislation and are now kicking the can further down the road on crucial Commons votes.
“Theresa May needs to face down the extreme Brexiteers in her party and start putting the national interest first.”
And Jo Stevens, a Labour MP backing the anti-Brexit Best for Britain group, said: “The government is running scared on key Brexit bills – and not for the first time.
“With less than a year until ‘Brexit day’ we’re still no closer to understanding what our future relationship with the EU would look like. Getting answers on the government’s plans is like getting blood out of a stone.”
The delay has prompted speculation that the withdrawal bill could be abandoned altogether and subsumed into the legislation to implement the Brexit deal – which must pass by next March.
The spokeswoman insisted there were “no plans” to do that, saying: “It is pretty normal for bills not to enter ping-pong immediately, and that is what has happened here.”
There is also no date for the return of either the trade or customs bills – when the Tory rebels plan to force a vote to keep the UK in the customs union.