The Liberal Democrats and Labour are plotting to delay Theresa May’s most important piece of Brexit legislation by months in a bid to destabilise her leadership.

Senior figures have told this newspaper they will block any attempt by the Tories to limit the amount of parliamentary debate on the Great Repeal Bill.

They hope to delay MPs green-lighting the law until the start of 2018, throwing the Prime Minister’s preparations for Brexit into uncertainty.

Talks between shadow cabinet ministers in both parties have already taken place and Tory MPs will be approached soon about backing the move.

The crucial vote is expected to come when MPs return in early September, just weeks before Mrs May faces her critics at the annual Conservative Party conference.

“This could be the thing that actually does it for Theresa May,” said a senior Lib Dem, referring to the possibility of forcing her to quit.