London - Prime Minister Theresa May's Brexit deal faced a likely defeat in an historic parliamentary vote Tuesday that risked pitching Britain into the unknown just 17 days before its scheduled split from the European Union.
Parliament's failure to back the divorce terms could unleash economic chaos: it raises the danger of Britain severing ties with its biggest trade partner on March 29 without a deal after 46 years of EU membership.
A hoarse-sounding May warned lawmakers in a last attempt to sway them that "Brexit could be lost" if they voted against her and said a no-deal Brexit would deliver a "significant economic shock".
May had dashed off to Strasbourg on Monday to wrest concessions from EU leaders in a last-gasp bid to win parliament's blessing in one of its most consequential votes in generations.
The British leader announced that she had secured the "legally binding changes" to the vexing issue of the Irish border that lawmakers had long sought.
But Attorney General Geoffrey Cox said the risk in the most contentious points of the 585-page withdrawal deal agreed with the EU "remains unchanged".
May's initial agreement with Brussels was crushed in January by an unlikely alliance of pro-European and Brexit-backing MPs in January.
Members of the European Research Group (ERG) of Brexit hardliners in May's Conservative Party and the Democratic Unionist Party (DUP) that props up her minority government both came out against the deal on Tuesday.
The main opposition Labour Party also urged parliament to vote down May's plan, making defeat almost certain.
The pound reversed the gains it had made after May's announcement on Monday and tumbled after Cox's advice was released.
'Nothing has really changed'
The so-called "backstop" solution for the Irish border -- designed to avert sectarian strife from returning to Britain's Northern Ireland -- is opposed by more fervent Brexit supporters.
They pressed May to secure the right for Britain to pull out of the arrangement or to make it time limited.
But Brussels has called it essential for preserving the bloc's external border after Brexit.
The DUP said in a statement that "sufficient progress has not been achieved at this time" and called for a "sensible deal".
The DUP's consent was seen as vital for swaying members of May's own party to soften their opposition and either back the deal or abstain in the vote.
While a few said they had changed their mind as a result of her last-minute changes, there was scant evidence of a significant shift in Brexiteer support.
'Brexit could be lost'
With just hours to go before the vote, May attempted to sway sceptics.
"I think everybody needs to recognise, for those who genuinely want to deliver Brexit, that actually if this deal does not go through tonight, then this house risks no Brexit at all," she said.
"The danger for those of us who want to deliver - to have faith with the British public and deliver on their vote for Brexit - is that if this vote is not passed tonight, if this deal is not passed, then Brexit could be lost," she said.
Leaders across Europe also united behind a message that this was the best and final offer Britain could expect.
"There will be no third chance," European Commission chief Jean-Claude Juncker said after his talks on Monday with May.
No-deal and delay
Another defeat would tee up additional votes on the way ahead that May has promised in a bid to preserve unity inside her fractured government.
One on Wednesday would decide whether Britain should simply leave on March 29 without any deal at all.
That option is fraught with economic dangers and is backed only by the most uncompromising proponents of the divorce.
The "no-deal" scenario's likely defeat would lead to a vote on Thursday on requesting a delay to Brexit from the EU.
The other 27 nations would need to back the extension unanimously and decide how long it should be. Their leaders will meet in Brussels for a summit on March 21-22.
But any postponement may have to be short-lived. Juncker on Monday said Brexit "should be complete before the European elections" at the end of May.
Meanwhile, German Chancellor Angela Merkel said Tuesday that the latest EU offer on Brexit was "clear and far-reaching", hours before the British parliament was set to vote on it.
"Today is an important day," Merkel said at a Berlin joint press conference with Belgian Prime Minister Charles Michel.
"I think that the 27 member states, represented by (European Commission chief) Jean-Claude Juncker and (EU chief negotiator) Michel Barnier, have again made clear and far-reaching proposals that take into account the concerns of Great Britain and to find answers for them.
"We want to get along well and we want an orderly exit by Britain, and now it's the task of the British parliament to reach decisions, and we'll be following closely what happens today, tomorrow and the day after in Britain."
Merkel added that "it's now up to the British parliament, so let's wait and see".
Michel called the latest EU offer "an outstretched hand" and voiced hope that British lawmakers would reach a "sensible" decision.