Brexit showdown vote on a knife-edge, a fresh twist emerges

A dramatic Brexit week concludes on Saturday with a vote in the House of Commons on Prime Minister Boris Johnson's revised deal to take Britain out of the European Union, though there could be yet another twist ahead with a key amendment designed to guard against a no-deal under consideration.

As hundreds of thousands of people rally outside parliament to demand another referendum, MPs inside will cast votes with a deal, no-deal or another delay all possibilities.

"There have been any number of false dawns. Deadlines for our departure have come and gone," Johnson wrote in The Sun on Saturday. "Today can be the day we get Brexit done.”

Three possible scenarios

1. Deal passes: A majority of MPs, probably 320, back the revised Brexit deal. Britain is on course to leave the EU on October 31 with an agreement, but legislation still needs to be rushed through the House of Commons and could be amended.

2. Deal rejected: MPs reject the revised Brexit deal, and a no-deal in a subsequent vote. The prime minister is compelled by law to ask the EU for an extension to the October 31 deadline. Johnson insists he will not countenance another delay, so it's uncertain how he would respond. A push for an election or a confirmatory referendum are possible.

3. Amended: A majority of MPs back an amendment withholding support for the deal until all relevant legislation is passed. This is an attempt to prevent a no-deal Brexit through the backdoor. Britain could still leave on October 31, but Johnson would be compelled by law to ask the EU for an extension to the deadline. Again, it's unclear how he would respond.

Johnson has the support of most Conservative MPs and a sprinkling of opposition lawmakers, but his deal is opposed by the Labour Party, Scottish Nationalists, Liberal Democrats and Northern Ireland's Democratic Unionists.

The prime minister appeared to have momentum behind him overnight in his bid to win the backing of 320 lawmakers, with several Labour MPs saying they would defy their party and skeptics within his own Conservatives also voicing support.

A hardcore of former Conservatives, hardline Brexiteers and Brexit-friendly opposition MPs are yet to declare their hands – persuading lawmakers in these three groups to back the deal will be key to passing the agreement.

The vote is expected to be extremely tight and even if Johnson wins, extensive legislation will need to be passed before October 31.

If the deal is rejected, a second vote would be held on a no-deal exit. If that is also rejected, the prime minister would be compelled by law to ask the European Council for an extension – a move Johnson has insisted he would not make.

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And the prime minister may yet be thrown off course by an amendment proposed by a group of former Conservative MPs along with Labour and the Liberal Democrats.

The planned amendment states that approval for the deal will be "withheld" until the legislation underpinning it is passed by through parliament.

It is designed to prevent the UK dropping out of the EU without a deal if the necessary legislation isn't in place before October 31, amid suspicions hardline Brexiteers may vote down the Withdrawal Agreement Bill in an attempt to force a no-deal.

If the amendment is approved, Johnson would be compelled to write to the European Council requesting another delay to Brexit and the central vote would become essentially symbolic.

The prime minister has repeatedly said he would not sanction an extension, however, and it is unclear how he would respond if his deal was defeated or the amendment passed.

It also remains uncertain whether the European Council would grant an extension. All 27 national leaders must agree to another delay, and French President Emmanuel Macron has indicated he could veto such a request.