Theresa May’s Brexit Bill is facing a stumbling block as peers from all major parties prepare a joint offensive to force the Prime Minister into making concessions.

Labour, Liberal Democrat and up to a dozen potential rebel Tory Lords are now increasingly confident they can make the Government do more to guarantee EU citizens rights and give Parliament more say over the final Brexit deal.

Senior Tories also admit that despite Downing Street’s desire for the bill to pass unchanged, ministers could find themselves outnumbered if opponents’ plans for altering Ms May’s bill are put to a vote.

Few Lords talk about blocking the Brexit Bill outright. But a failure by the Government to address the peers concerns raised in a two-day debate starting Monday, could well delay a Brexit timetable already being stretched towards the limits Ms May set herself.

One senior Tory member of the Lords told The Independent: “The challenge we have is between those colleagues who want to make the agreement better, who want to think long and hard about the negotiation, and those colleagues who are saying it’s time to fight back.

“What we can do is refine and make the bill better, but what we can’t do – and I know there are colleagues who want to do this – is undo the decision of the referendum.”

The peer said: “There will be areas where the Government may give way and one could be on EU citizens rights. But the more reasonable the Lords are in accepting the referendum, the more reasonable the Government will be in working with some of the amendments.”

The upper house, where the Conservatives are in a minority, comes to debate the bill after it passed through the Commons unchanged earlier this month with the support of Tories and most of Jeremy Corbyn’s group of Labour MPs.

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But Labour and the Liberal Democrats in the Lords have tabled a string of amendments to the legislation, including one supported by Conservative peer Patience Wheatcroft, who is said to represent a handful of rebellious Tories.

Another Tory Lord said: “There are going to be amendments put down and there is going to be cross-bench support for some of them.”

Among changes sought by the Liberal Democrats is the amendment to guarantee the rights of EU citizens already living in the country before the Brexit process begins.

Party Leader Tim Farron told The Independent: “We will make protecting these citizens’ rights a top priority. We have tabled amendments in the Commons on this matter and we will do so again in the House of Lords.

“We believe we have support on all sides and I urge the Prime Minister to not force a fight and to accept the Liberal Democrat amendment.”

A Downing Street source said the Prime Minister had already signalled a desire to secure the rights of EU citizens in this country and those of British citizens living in Europe, but there has been a question over when and how it can happen.

Guy Verhofstadt says he's received thousands of letters from Brits wanting to remain EU citizens

Whispers that Ms May could be amenable to doing something on the issue in the face of a strong Lords challenge, and has even put out feelers to the Lib Dems, persist. But officially No 10 still insists it should not be contemplated as part of the legislative process around Article 50.

Labour peers have followed the footsteps of the party’s MPs in the Commons in tabling an amendment to secure a “meaningful” Commons vote on the final deal that Ms May secures.

The Commons amendment was not passed, though shadow Brexit Secretary Keir Starmer claimed a victory of sorts after receiving a verbal assurance that MPs would get to approve Ms May’s final Brexit deal before the European Parliament does, thus allowing time for a potential renegotiation of terms if Westminster politicians do not like it.

But Labour Lords believe they may have more luck at locking the ‘meaningful vote’ measure into statute, with the support of cross-bench peers like Lord Pannick, who was involved in the Article 50 Supreme Court case.

A party source said: “The amendment is backed by Lord Pannick, one of the most respected peers in the House, who should be able to rally cross-bench support.”

Any votes on amendments will take place after the two days of debate at a later point in the Lords stages of the bill.

The Government had originally signalled it wanted all stages of the bill completed by 7 March, opening the way for Ms May to potentially trigger Article 50 at the EU Summit on 9 March.