THERESA May's flagship Brexit legislation is 'constitutionally unacceptable' and will need to be substantially rewritten, an influential Bolton peer has said.

The European Union (Withdrawal) Bill, which will be debated by peers for the first time on Tuesday, has been heavily criticised in a damning report by the Lords Constitution Committee, chaired by Baroness Taylor of Bolton.

The peers said the Bill was 'fundamentally flawed' in multiple ways and risked 'undermining legal certainty'.

The Government wants the bill to transpose rules and regulations from Brussels into domestic law in time for Brexit.

But the Lords Constitution Committee said the task was complicated not only by its 'scale and complexity' but also because 'in many areas the final shape of that law will depend on the outcome of the UK's negotiations with the EU'.

Baroness Taylor 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."

A Department for Exiting the European Union spokesman said: "The purpose of this Bill is to ensure the UK has a functioning statute book on exit day. It is an essential piece of legislation in the national interest.

"This is a long and detailed report, and we will consider it carefully.

"From the beginning we have been committed to working collaboratively with Parliamentarians to improve the Bill wherever possible. We will continue to do so in the Lords."