Downing Street has warned Lords that an attempt to give Parliament the final say over Brexit will “incentivise” the EU to give Britain a bad deal.

The Prime Minister’s spokesman argued that guaranteeing Parliament the power to reject Theresa May’s deal would “give strength” to EU negotiators during talks.

He made the claim ahead of a critical vote in the Lords tomorrow, where many peers are planning to defy the Prime Minister and demand it be written into law that Parliament must approve the deal she secures.

A group of Tory Lords, including Michael Heseltine, are also believed to be among those ready to vote against the Government on the matter, being addressed as part of the debate on Ms May’s Bill to trigger Article 50.

Asked for Ms May's message to peers preparing to vote, her spokesman said: “She believes we should not commit to any process that would incentivise the EU to offer us a bad deal.

“If we are in a position where any deal negotiated by the Prime Minister could be rejected by MPs, that gives strength potentially to other parties in the negotiation.”

The House of Lords is expected to vote on Tuesday on an amendment calling for Westminster to be given a “meaningful” vote on the withdrawal agreement secured by the Prime Minister.

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Ms May has given verbal assurances that Parliament will have a vote on the deal she secures, but only on a “take it or leave it” basis which would still see the UK crash out of the EU without a deal if MPs reject her agreement.

Critics have claimed Ms May’s pledge is an attempt to push Parliament into backing her deal, by threatening to leave with no deal and trade on WTO rules if it is rejected, something which could harm the UK economy.

Instead, peers are insisting they should be given the option of telling Ms May and her ministers to go back to the EU and renegotiate a better deal if Parliament rejects her first effort.

Lord Heseltine has already said he would vote against the Government on the matter because he believes that public should have a chance to change their minds on Brexit.

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Gina Miller, the campaigner who fought and beat the Government in the supreme Court Brexit case, has said the judgment she won and separate legal advice demand the Government would need a separate act of Parliament before taking the EUK out of the EU with no deal.

Peers have already given Ms May a bloody nose over guaranteeing the rights of EU citizens’ rights living in the UK before Brexit, voting last week for a unilateral guarantee to be included in the Article 50 Bill.