LONDON (Reuters) – British Prime Minister Theresa May called on parliament to support legislation to sever political, financial and legal ties with the European Union, a step towards Brexit the opposition says it will challenge.

The repeal bill, or EU withdrawal bill, is central to the government’s plan to exit the bloc in 2019, untangling Britain from more than 40 years of EU lawmaking and repealing the treaty that first made Britain a member in 1972.

On Thursday, parliament will start debating the “main principles” of the bill, which seeks largely to copy and paste EU law into British legislation. Lawmakers will vote on Monday on whether the bill can move on to the next part of its long legislative journey, when it could be amended.

Its safe passage through parliament is especially important for a government that has been criticised in Brussels over its Brexit strategy and after a series of leaks, one of a letter to companies asking them to support Britain’s plans.

“The repeal bill helps deliver the outcome the British people voted for by ending the role of the EU in UK law, but it’s also the single most important step we can take to prevent a cliff-edge for people and businesses, because it provides legal certainty,” May said in a statement.

“We’ve made time for proper parliamentary scrutiny of Brexit legislation, and I look forward to the contributions of MPs (lawmakers) from across the House (of Commons). But that contribution should fit with our shared aim: to help get the best Brexit for Britain.”

Her Brexit minister, David Davis, also urged lawmakers to speak out if they felt that any rights were not carried forward into British law by the bill, a challenge to the main opposition Labour Party which has said it cannot support the legislation without it being amended to better protect workers’ rights.

Monday’s vote will be a test of May’s deal with a small Northern Irish Party to shore up the majority she lost in an election she did not have to call in June. Labour would need to convince EU supporters in May’s Conservatives to side with them to vote down the bill.

Some more vocal pro-EU lawmakers in the Conservative Party have now said they will vote with the government after asking for reassurance that parliament will be able to scrutinise any changes to the law.

“We are not rejecting EU law, but embracing the work done between member states in over forty years of membership and using that solid foundation to build on in the future, once we return to being masters of our own laws,” Davis said.

“I hope everyone in this House recognises this bill’s essential nature – it is the foundation upon which we will legislate for years to come – and I look forward to working with the whole House to deliver the bill,” he said in a statement.

But Chris Leslie, a pro-EU campaigner and Labour lawmaker, questioned whether parliament could trust May and Davis.

“(The bill) will give the executive unparalleled powers to change laws that affect the lives and rights of the British people by the stroke of a ministerial pen,” he said.

“For Theresa May and David Davis to ask us to just trust them isn’t good enough. I urge MPs from all parties to vote down this shambles of a bill.”