Prime Minister Boris Johnson faces two crucial Brexit votes in parliament on Tuesday that will determine if he can fulfil his "do or die" promise to take Britain out of the European Union next week.

As MPs threatened to derail the ratification of his Brexit deal, Johnson warned that defeat would see him abandon the legislation -- and instead seek to call a snap election.

Britain is entering a cliffhanger finale to a drama sparked by the 2016 referendum vote on whether to leave the EU, which has plunged the country into three years of political turmoil.

Johnson was forced on Saturday to ask EU leaders to postpone the October 31 deadline for leaving -- something he once said he would rather be "dead in a ditch" than do.

He was required by law to send the letter after MPs refused to back the divorce deal he struck with Brussels last week, which paves the way for an orderly end to 46 years of integration.

However, he still has a chance of avoiding a delay if he can get legislation implementing the treaty through parliament by October 31.

The House of Commons will hold its first vote on the bill at around 1800 GMT, followed by another vote on Johnson's timetable motion to rush it through parliament ahead of Brexit day.

Opening the debate, Johnson urged MPs to support the legislation so "we can get Brexit done and move our country on".

He warned that if they failed, the "bill will have to be pulled... and we will have to go forward to a general election," adding: "I will argue at that election: let's get Brexit done."

Two previous attempts to call an election have failed, however.

"What on earth will the public think of us if this House again tonight votes not to get on with it, not to deliver Brexit on October 31 but to hand over control of what happens next to the EU?" the Conservative leader said.

Johnson warned defeat would kill any hope of leaving the EU with a deal on October 31, and risks a "no deal" exit if the EU declines to approve a third Brexit delay.

However, Labour leader Jeremy Corbyn warned the prime minister was trying to "blindside" parliament into supporting a substandard deal.

"A deal and a bill that fails to protect our rights and our natural world, fails to protect jobs and the economy, fails to protect every region and every nation in the UK," he said.

- EU 'will be ready' -

European Council President Donald Tusk said the other 27 EU leaders were mulling Johnson's request to delay Brexit, but it would depend on how MPs vote.

"It is obvious that the result of these consultations will very much depend on what the British parliament decides, or doesn't decide," he told the European Parliament.

"We should be ready for every scenario."

He added on Twitter: "I made clear to PM @BorisJohnson: a no-deal #Brexit will never be our decision."

MPs voted to demand Johnson delay Brexit to try to avoid the damage of a "no deal" exit, where Britain severs ties with its closest trading partner with no new plans in place.

The prime minister on Tuesday said that if they backed his bill, "we can de-escalate those no-deal preparations immediately".

The deal is the second of its kind, after a Brexit text agreed by his predecessor Theresa May was rejected three times by MPs earlier this year.

Outgoing European Commission President Jean-Claude Juncker said Tuesday that the bloc had "done all in our power" to assure an orderly divorce.

- Years of trade talks -

Even if Johnson wins the two votes on Tuesday evening, MPs could still derail his bill.

Some lawmakers want to secure much closer future trade relations with the EU after Brexit, seeking to amend the bill to demand Britain stay in the bloc's customs union.

After tens of thousands of people demonstrated in London on Saturday in favour of a second referendum, some MPs will also seek to attach plans for a "People's Vote" to the legislation.

If the Brexit withdrawal bill passes the Commons unscathed, it must still be approved by the unelected upper House of Lords -- and then the European Parliament.

The withdrawal deal covers EU citizens' rights, Britain's financial settlements, a post-Brexit transition period until at least the end of 2020 and new trade arrangements for Northern Ireland.

But it only sets out the broad parameters for Britain's future ties with the EU, and EU Brexit negotiator Michel Barnier warned this could be a much lengthier process.

"We will have to renegotiate for one year, two years, three years, maybe more in some areas, to rebuild all that will have been pulled apart by the desire of those backing Brexit," Barnier said.