Theresa May has defiantly insisted her timetable for triggering Brexit will not be blown off course despite suffering her first Parliamentary defeat over the Article 50 bill.
The House of Lords voted to amend the Bill to force the Government to guarantee the rights of EU citizens living in the UK. Seven Tory peers, including former pensions minister Baroness Altmann, backed the amendment.
But the Prime Minister is confident the amendment will be rejected by the Commons this month, and Downing St insisted the timetable for Brexit "remains unchanged".
Lords who voted to alter the Bill were accused of "playing with fire" and critics accused them of pointless "posturing" and "doing a disservice to the national interest".
The scale of the Government's defeat in the Lords, where the proposal to amend the Bill was passed by 358 votes to 256, prompted speculation that May could face a fresh Tory rebellion when the Bill returns to the Commons.
Conservative whips are confident, however, that no more than a handful of Tory MPs will support the amendment.
Labour's amendment to the EU (Notification of Withdrawal) Bill, tabled with Liberal Democrat and crossbench support, calls for ministers to bring forward proposals ensuring the rights of EU citizens in the UK continue post-Brexit.
Jeremy Corbyn, the Labour leader who ordered his MPs to support the Bill unamended when it went through the Commons, described the Lords vote as "great news", raising the possibility that he might tell his MPs to back the amendment in the Commons.
He said: "The Government must now do the decent thing and guarantee the rights of EU citizens living in the UK."
The Lords will vote next week on a further amendment that would give MPs a "meaningful vote" on the outcome of May's negotiations with the EU - a vote that could also go against the Government.
The Prime Minister could then come under pressure from her own MPs to agree to that amendment passing into law.
The amended Bill will return to the House of Commons on March 13 and 14, when MPs will debate whether to keep the changes.
May intends to notify the EU of Britain's intention to withdraw on March 15, triggering two years of negotiations that would end with Brexit in 2019.
The Prime Minister has already told MPs she wants to protect the rights of the 3 million EU citizens living in the UK, but will only issue that guarantee once the EU has granted reciprocal rights to the 900,000 Britons in EU member states.
Iain Duncan Smith, the former Tory leader who voted to leave the EU, said of Wednesday night's vote: "It is a bit of posturing by some people in the Lords who are beating their chests and puffing out their own sense of self-importance.
"My answer to that is 'jolly good, nice to hear from you, but it has no bearing on Theresa May's Article 50 negotiations'.
"It speaks volumes about who some of these people in the Lords think they are and absolutely nothing about Brexit."
John Penrose, the former Conservative Constitution minister, said: "The Commons voted decisively that the rights of Brits abroad are just as important as the rights of EU nationals living here.
"People will find it hard to understand why the unelected House of Lords thinks that is wrong."
Baroness Smith, the Labour Leader of the Lords, has already made it clear peers are unlikely to fight further if MPs vote down their amendment.
This should allow May to declare on Wednesday March 15 that the Bill has received its royal assent and negotiations about Brexit can begin.
Lord Lamont, the former Tory chancellor, said of the Lords vote: "I think they are doing a disservice to the national interest. You shouldn't be imposing conditions [on the Government].
"I hope that it will be rejected by the Commons and sent back here. If that is not the end of the matter I think the Lords will be playing with fire."
Sir Bill Cash, the chairman of the European Union scrutiny committee, said he was confident that MPs will overturn the amendment.
He said: "It is outrageous that these unelected people should undermine the vote in the House of Commons and the rights of British voters who live abroad."
Tory peers including Lord Hogg and Lord Bowness spoke in favour of the amendment under the glare of Amber Rudd, the Home Secretary, who wrote to all peers on Tuesday urging them not to stand in the way of the Brexit bill.
She watched the debate from the steps in front of the Queen's throne in the Lords. During the debate that preceded the vote, Lord Tebbit, the former Cabinet minister, accused fellow peers of putting "the rights of foreigners" above those of British people as he implored them not to back the amendment.
He asked the House of Lords: "Why is everybody here today so excited about an amendment that looks after the foreigners and not the British?"
He was jeered by some peers as he referred to EU citizens in the UK as "foreigners", but some on the Tory benches shouted "quite right" as he made his point.
He said the proposed amendment - to guarantee the rights of EU migrants living in the UK - would weaken May's power to negotiate for the rights of British citizens living in the EU once Article 50 is triggered.
Lord Tebbit, who has a Danish son-in-law living in the UK and a son who has lived and worked in Germany, said: "It seems to me that the first duty of this Parliament of the United Kingdom is to care for the interests of the citizens of this kingdom.
"If we are to be concerned about anybody's rights after Brexit, to live anywhere on this continent of Europe, it should be our concern for the rights of British people to live freely and peacefully in those other parts.
"Somehow or other today we seem to be thinking of nothing but the rights of foreigners."
The former Tory chancellor Lord Lawson, who lives in France, said it was "quite clear" there is "no danger whatever to EU citizens in the UK" and said it was "wholly deplorable" that the amendment had been tabled.
Passing it would only "stir up fear and concern among the EU residents in this country that they may not be able to stay when there is no question they will be", he said.
Lord Bragg, the Labour peer and broadcaster, said Brexit would be a "disaster" and suggested EU citizens living in the UK had been "reduced to pawns in a Government tragedy".
Despite the presence of Rudd, Viscount Hogg, the Conservative peer and former agriculture minister, defied his party by speaking in favour of the amendment.
He said it "offends natural justice" to deny the rights EU citizens and called on the Government to "take the moral high ground" in the Brexit negotiations.
Fellow Tory Lord Bowness said: "We are not dealing with enemy aliens in times of war. Let us give the assurance and show that we are indeed the generous outward-looking country we are heading for in these Brexit negotiations."
A spokesman for the Department for Exiting the European Union said: "We are disappointed the Lords have chosen to amend a Bill that the Commons passed without amendment.
The Bill has a straightforward purpose - to enact the referendum result and allow the government to get on with the negotiations.
"Our position on EU nationals has repeatedly been made clear. We want to guarantee the rights of EU citizens who are already living in Britain, and the rights of British nationals living in other member states, as early as we can."