David Cameron's hopes of averting another embarrassing revolt by his own MPs over immigration appeared to have been dashed after the rebels dismissed the latest government concessions.
Home secretary Theresa May tabled a last-minute amendment to the keynote Immigration Bill before MPs which would allow the government to strip terrorist suspects of their UK citizenship.
But as the crucial all-day debate got under way in the Commons, there were clear signs that the rebel Tories demanding tougher controls were not going to back down, with claims that the concession failed to answer their concerns.
And that has raised the real risk of the government being defeated on the issue and of pitching the Immigration Bill into chaos as ministers decide how to proceed.
The rebels want the government to take the power to deport immigrant criminals even if suspects use human rights laws to claim they have a family life in the UK and should not be deported.
Claims that the move would put Britain on a collision course with the European Court of Human Rights in Strasbourg have been dismissed by the rebels who insist it is perfectly legal under the rules.
The Tory MP who tabled the rebel amendment, Dominic Raab, said: "My amendment is legal, it is workable and it ought to be passed. And I think it will pass. I do not think the government will want to vote against it."
The prime minister's spokesman would not be drawn on whether, if all else failed, ministers would be prepared to back down and vote in favour of the amendment.
The calculation is a pretty simple one. Either ministers stand firm and hope to defeat the rebels with attempts to win over more through further concessions and all the usual warnings and threats about the dangers of disunity.
Or they cave in and decide to allow the amendment to go through and deal with any possible legal consequences later.
The great danger for Cameron is that a defeat, coming in the weeks before the Euro elections, would present a disunited and squabbling party to the electorate just when it appears Nigel Farage's Ukip is heading for victory.
It would be a mistake to underestimate the determination of the rebels who have previously defeated the government.
Two years ago, 81 voted against the government to force a European Referendum Bill into parliament. And last year, enough opposed Cameron's call for air strikes on Syria to defeat the move.