THE European Union (Withdrawal) Bill is “fundamentally flawed from a constitutional perspective” and will need to be substantially rewritten, a Lords report has said as peers prepare to debate the legislation today and tomorrow.

The Lords constitution committee said that the bill as it stood risked “undermining legal certainty” and should be significantly changed, even though it has already been voted through the House of Commons.

It will be subject to fierce debate in the Lords over the next 48 hours, with Remain-supporting peers expected to vote for a motion of regret that the public is not getting another say over Brexit.

The purpose of the legislation is to transpose EU legislation into domestic law in time for Brexit day at the end of March 2019.

But the Lords committee said the task was complicated not only by its “scale and complexity”, but problems with the drafting.

“We conclude that the bill risks fundamentally undermining legal certainty in a number of ways,” the cross-party group said.

The peers said the method proposed to create a new category of “retained EU law” would cause “problematic uncertainties and ambiguities”.

They also criticised the elements of the bill granting ministers power to amend regulations without full parliamentary scrutiny, known as Henry VIII powers, saying they are “overly broad” and there is an “unacceptably wide” emergency procedure for short-term changes.

Thirdly, the peers warned that failure to secure agreement from the Scottish and Welsh administrations about the devolution of powers returned from Brussels could have “significant constitutional repercussions”.

“The bill is therefore fundamentally flawed from a constitutional perspective in multiple ways,” the report said.

Lady Taylor of Bolton, the chairwoman of the committee, said: “We acknowledge the scale, challenge and unprecedented nature of the task of converting existing EU law into UK law, but as it stands, this bill is constitutionally unacceptable.”

All of the aspects of the bill criticised by the peers have already been challenged in the Commons, but ultimately the government was only defeated on one issue, as MPs insisted they wanted a guaranteed vote on the terms of leaving before Brexit.

Labour is planning to support about 20 amendments in the Lords, meaning there is a chance of government defeats with the support of crossbench and LibDem peers.

Top of the demands are a role for parliament in the event of no deal, a vote for the UK Parliament on the final deal, and limitations on Henry VIII powers.

Angela Smith, the Labour leader in the Lords, said: “We will work with others around the Lords to change this bill so it becomes an enabling measure for, rather than an obstacle, to a transitional period ... this will include seeking amendments relating to ‘exit day.’”

Scottish Brexit minister Mike Russell and his Welsh counterpart Mark Drakeford, last night met with peers in Westminster to discuss amendments to the bill to protect devolution. Russell said: “We have made clear we are unable to recommend consent to the EU Withdrawal Bill in its current form.

"That’s because it disregards the devolution settlement by allowing the UK Government to take control of clearly devolved policy areas like farming and fishing. The Scottish and Welsh governments published amendments to the bill ... to fix this issue. Unfortunately the UK Government chose not to accept those amendments or to meet their commitment to bring forward their own amendments in the House of Commons.”