Home Secretary Theresa May has defended her insistence that the status of EU nationals living in the UK must be part of the Brexit negotiations after a furious backlash from Tory MPs.
Conservative MPs lined up in the Commons to condemn her comments, accusing Mrs May - seen as the frontrunner in the race to succeed David Cameron - of a "catastrophic error of judgment".
But appearing before Conservative MPs at a hustings for the leadership contenders, she took on the issue head-on insisting the Government had to be able to defend the position of British nationals living in the EU.
The row erupted as Boris Johnson gave his backing to Energy Minister and fellow Leave campaigner Andrea Leadsom in the leadership race.
The former London mayor - who saw his own leadership hopes dashed following a devastating personal attack by Michael Gove, the other lead figure in Vote Leave - pointedly praised Mrs Leadsom as "level-headed, kind (and) trustworthy".
"Andrea Leadsom offers the zap, the drive, and the determination essential for the next leader of this country," he said in a statement.
With MPs set to cast their votes on Tuesday in the first round of balloting, a poll by the ConservativeHome website of 1,214 party members put Mrs Leadsom narrowly ahead with 38%, one point ahead of Mrs May on 37%.
The Home Secretary used her hustings address to argue that the Government could not afford to "give away" its negotiating position when it came to the status of British ex-pats in the EU.
"It is about a very logical, correct, practical procedure," a spokesman said.
Earlier in the Commons, Immigration Minister James Brokenshire faced an onslaught from furious Conservative MPs as he denied the Government was treating EU nationals in the UK as a "bargaining chip".
Replying to an urgent question, he said it would be "unwise" to fully guarantee that EU citizens could stay in the UK without the 1.2 million Britons living in EU countries receiving the same assurances.
Tory MP Anne Main, who backed Brexit, said Mrs May should not have questioned the right of EU nationals already in the UK to remain.
"This has been raised by the Home Secretary and it is a catastrophic error of judgment for someone who wishes to lead this country to even suggest those people who are here legally, working with families and settled, should be even part of the negotiations," she said.
"(She) has made a real big error of judgment and that message needs to go back to the Home Secretary today."
The chairman of the Commons Treasury Committee Andrew Tyrie said that delay was not a "realistic option" and ministers should "just get on" and confirm the status of EU nationals.
"Protecting their rights is the only ethical position that can be taken and what's more, the longer the uncertainty about this question persists the greater the risk of the economic downturn and economic consequences," he said.
Sarah Wollaston, the Tory MP who chairs the Health Select Committee, expressed concern about the position of the 55,000 NHS workers qualified elsewhere in the EU along with 80,000 members of the care sector.
"They need security not just now but in the long term because the workforce crisis is one of the biggest challenges facing the NHS," she said.
Mr Brokenshire stressed there would be "no immediate" change in the status of EU citizens living in the UK and the Brexit negotiations would reflect the "immense contribution" they made to country.
However it was also the duty of the Government to secure the interests of the 1.2 million British citizens living and working elsewhere in the EU.
"It has been suggested the Government could now fully guarantee EU nationals living in the UK the right to stay," he said.
"This would be unwise without a parallel assurance from European governments regarding British nationals living in their countries.
"Such a step might also have the unintended consequence of prompting EU immigration to the UK."
Mrs May goes into the first round of voting with a clear lead among MPs but the contest will be decided by grassroots members throughout the country.
She is joined on the ballot paper by Mrs Leadsom, Mr Gove, Work and Pensions Secretary Stephen Crabb and former defence secretary Liam Fox.
The candidate who finishes last will be eliminated and others may choose to drop out if they feel they have no chance of succeeding.
Further votes of MPs will be held - with the next due on Thursday - until the candidates are whittled down to a shortlist of two who will go forward into the final postal ballot of the entire party membership.
A YouGov poll of The Times of 994 Conservative Party members suggested that if it came to a final round run-off between Mrs May and Mrs Leadsom, the Home Secretary would win by 63% to 31%.