Theresa May faces a battle to get a key piece of Brexit legislation through Parliament, opponents have warned.
Shadow Brexit Secretary Sir Keir Starmer said it was “highly likely” Labour would seek to amend the Repeal Bill, which aims to convert EU legislation into British law.
The SNP, Lib Dems and Green MP Caroline Lucas will also press for changes.
The bill, described by the PM as an “essential step” to EU withdrawal – was the centrepiece of the Queen’s Speech.
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It will repeal the 1972 European Communities Act, which took Britain into the EU and meant that European law took precedence over laws passed in the British parliament.
It starts its Parliamentary journey next week but it is not likely be debated on the floor of the Commons until the autumn, with some predicting fireworks as MPs on all sides bid to change Mrs May’s approach to Brexit in a series of votes.
Last month, 49 Labour MPs defied party leader Jeremy Corbyn by backing a Queen’s Speech amendment calling for Britain to stay in the single market and customs union after Brexit.
Conservative backbenchers opposed to what they regard as a “hard Brexit” are also reported to be plotting to force changes to the Repeal Bill.
Labour rebels are understood to be planning to join forces with Tory rebels, the Lib Dems and the SNP to force changes to the Repeal Bill, and future pieces of Brexit legislation.
The Labour leadership backs an exit from the single market but wants to protect EU safeguards on employment rights and the environment – and is concerned these could be scrapped or watered down by the Repeal Bill.
Speaking to BBC Radio 4’s Today programme, Sir Keir said the Labour leadership was likely to table amendments that would seek to curb the power of government ministers to amend or scrap EU laws without MPs’ scrutiny.
He said: “I think it’s highly likely we will want to table that push amendment dealing with the scope of power given to the executive but also to concentrate on issues such as enforcement of rights and protections.”
Green Party co-leader Caroline Lucas told the programme she was also preparing to amend the Bill, while the SNP’s Stephen Gethins said his party “had a responsibility” to work with others over the terms.
Former Brexit minister David Jones, who lost his frontbench role in the post-election reshuffle, said Labour should back the government over the Bill after promising to deliver Brexit in its manifesto.
Pressed on the scale of the challenge, he told the Today programme: “Clearly there is huge pressure, though, having said that, I have no doubt that we will be able to do it.”
Of 27 Bills and draft Bills in the Queen’s Speech, eight were devoted to leaving the EU.
In addition to the Repeal Bill, there were separate pieces of legislation on on customs, trade, immigration, fisheries, agriculture, nuclear safeguards and the international sanctions regime.