Boris Johnson has led a Tory rebellion against the Government's failure to guarantee the post-Brexit rights of UK-based EU nationals, as MPs backed a motion demanding a rethink.
The prominent Leave supporter was among five Conservatives who supported Labour's call for ministers to "commit with urgency" to giving the estimated three million EU nationals in the UK the right to remain.
Home Secretary Theresa May has faced fierce criticism over her refusal to give firm assurances that EU citizens living in the UK will be allowed to stay, having suggested the issue will play a part in negotiations with Brussels.
Mr Johnson teamed up with shadow home secretary Andy Burnham to criticise the Government's position, insisting it was "very disappointing" that the status of EU nationals has been called into question.
His Tory colleagues who backed the non-binding motion included former London mayoral candidate Zac Goldsmith, Richard Fuller (Bedford), Tania Mathias (Twickenham) and Philip Hollobone (Kettering).
Labour forced the motion to a vote and it was backed by 245 votes to two, which included support from the SNP, Plaid Cymru plus Green Party MP Caroline Lucas and Ukip's Douglas Carswell.
An analysis of the division list showed Ulster Unionist Party MPs Tom Elliott (Fermanagh & South Tyrone) and Danny Kinahan (Antrim South) voted both aye and no, which is regarded as an abstention.
During the debate, Mr Burnham suggested Home Secretary Mrs May was trying to "woo" the Tory Party grassroots as she fights to become the next prime minister and of using EU citizens as "bargaining chips".
He attacked Mrs May for failing to attend the debate and accused her of an "abdication of leadership".
"One could only speculate that she made these comments in a bid to woo the grassroots of the Tory Party in her current situation," he said.
Meanwhile, leading Brexit campaigner Mr Johnson said: "I'd like to set on record what I think has been said already, that countless times the Vote Leave campaign gave exactly this reassurance to everybody from other EU countries living and working here and it is very, very disappointing that this should be called into question.
"I think it is absolutely right to issue the strongest possible reassurance to EU nationals in this country."