A leading Tory Brexit rebel has branded the Government’s plan to fix the date of the UK’s EU withdrawal in law as “incoherent and thoroughly stupid”.

Dominic Grieve said he would not support the proposal brought forward by ministers, raising the prospect the Government could suffer an early defeat on Theresa May’s key piece of Brexit legislation.

Ministers hoped the proposal might allay some concerns about so-called Henry VIII powers in the bill, as it serves to clarify when they will expire – but Mr Grieve said it tied the Government’s hands.

He told Sky News: “The amendment being tabled by the Government saying there is a day on which we have got to go, which seems to reduce the Government’s flexibility in its negotiations … is an incoherent and thoroughly stupid amendment and it won’t have my support.”

The ex-Attorney General added: “I was more than a bit mystified by the Government’s tabling of its amendment on Friday setting a fixed date by Parliament when we have to withdraw because I find it impossible to understand how that can be helpful to anyone. I’m going to need a great deal of persuasion that that amendment has any merit at all.”

Ministers had said last week they hoped the amendment would show “not only are we in listening mode, but actually that we are prepared to clarify and adapt where necessary”.

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One told The Independent: “The exit day change should give people peace of mind about the sunsetting clauses that are in the bill [relating to the Henry VIII powers], showing that they operate around a date and will definitely expire around March 2021.”

But MPs who backed Remain in the Tory referendum would rather have seen the issue of Herny VIII powers – which will allow the Government to change laws without full parliamentary scrutiny – dealt with without a date for Brexit enshrined in law.

Instead the Government’s amendment is being seen as a way of ingratiating ministers to Tory Eurosceptics, who want to ensure Brexit takes place in March 2019 and no extension of negotiations occurs.

Mr Greive said he and other MPs are having “a sensible discussion” with the Government on how to change the bill and he was confident a compromise could be reached.

A Downing Street spokesman said: “The PM was simply setting out that that’s the date the UK will be leaving the EU and it makes sense that the authority of EU law in Britain ends at exactly the same time.”

More generally Mr Grieve suggested that Brexit was only inevitable “unless the electorate changes its mind”. He went on: “My view has always been that unless or until one can see that that’s happened the Government is really duty bound to try to implement it.