The House of Lords looks set Wednesday to defy Prime Minister Theresa May by demanding guarantees for EU nationals living in Britain, delaying a bill she needs to start Brexit negotiations.

The opposition Labour party is hopeful it can push through an amendment to legislation empowering May to trigger Article 50 of the EU's Lisbon Treaty, which begins two years of divorce talks.

May hoped the bill would pass through parliament by next week and has promised EU leaders to formally start the withdrawal process by the end of the month.

But if peers in the upper house of parliament vote to amend the bill later Wednesday, it must return to MPs in the House of Commons for further deliberation.

The potential setback for May comes after former prime minister John Major accused her of "cheap rhetoric" and an "over-optimistic" view of Brexit.

Scottish First Minister Nicola Sturgeon also this week warned she may be forced to call for a new independence referendum to defend Scotland's rights.

- 'Deeply disappointing' -

Immigration promises to be a critical issue once negotiations with Brussels begin.

Eight months after Britons voted to leave the European Union, the fate of more than three million EU nationals living in Britain remains uncertain, causing intense worry for those concerned.

May has said she has every intention of guaranteeing their right to stay, but argues she cannot move unilaterally without securing similar assurances for the 1.2 million British citizens living in the 27 other EU countries.

But Labour's leader in the Lords, Baroness Angela Smith of Basildon, said the government's response so far had been "deeply disappointing".

"To continue to use people as bargaining chips in this way is not only shameful but could have a dire impact on the UK's economy and essential services," she said.

The Labour amendment demands the government bring forward proposals within three months to protect the rights of EU and European Economic Area citizens legally resident in Britain.

A similar amendment failed to pass the Commons when the bill was debated there last month, but May's Conservative party is in the minority in the 800-seat House of Lords.

Labour expects support from members of the pro-European Liberal Democrats, independent peers and potentially some Conservative rebels.

Lord Newby, the leader of the Liberal Democrats in the Lords, said: "This is a vital amendment that will put pressure on MPs to see sense."

- 'Issue of timing' -

May had hoped to agree a deal on EU nationals last December, but EU leaders insisted that on this subject, as with any other, there can be no talks until after the government triggers Article 50.

"The hold-up is less an issue of principle than one of timing," Home Secretary Amber Rudd wrote in a private letter sent to all peers, obtained by AFP.

"However, I believe the UK and the EU have a common goal to provide this assurance as quickly as possible."

The government has promised that parliament will be able to vote on new immigration arrangements after Brexit.

However, key issues such as the cut-off date for EU arrivals covered under the new arrangements are likely to form part of the Brexit negotiations.

The government risks defeat on another amendment to the Brexit bill due for consideration next week, on giving parliament a vote on the final withdrawal deal.

May has given verbal assurances that such a vote will take place, but her opponents want this enshrined in law. afp