Pro-Europe lawmakers' efforts to temper Theresa May's Brexit plan gained traction when a member of her own party broke ranks to criticise her strategy before parliamentary votes that will test her government's slim majority.
A three-day debate on a law giving May the right to trigger Britain's exit from the European Union begins on Monday, and will be followed by a series of votes on whether to attach extra conditions to her plan to start talks by March 31.
Lawmakers voted overwhelmingly in favour of the principle of the new law last week, signalling May is unlikely to be blocked outright from triggering Brexit and she will need the support of her whole party to approve the strategy without change.
Sunday brought the first signs of internal Conservative Party dissent which, if they spread, could see the law amended, damaging May's authority domestically and potentially giving EU negotiators a powerful lever in the exit talks.
"We could be faced with the prospect of leaving the EU by 'falling off a cliff' - as some have described leaving with no deal - with potentially disastrous economic consequences," MP Neil Carmichael said on Sunday.
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"Parliament must have a final say when we get to the end-game."
Carmichael is the most outspoken of a handful of critics within May's party who could join forces with opposition MPs from the Labour Party and the Scottish National Party to defeat the government.
May has a 16-seat working majority, but the actual number of rebels needed to inflict defeat could be higher depending on how smaller parties and pro-Brexit Labour MPs vote. Upwards of 20 Conservative votes might be needed to defeat her.
