British Prime Minister Boris Johnson's proposed Brexit deal was facing a key test in parliament on Saturday. 

The Conservative Party leader is attempting to win support for the latest Brexit pact he struck in Brussels on Thursday, in which the UK and the EU had agreed on the terms of a new withdrawal agreement. 

Addressing lawmakers in the House of Commons, Johnson said they had a historic opportunity to endorse a "new way forward" for Britain and the EU.

"I hope ... that this is the moment when we can finally achieve that resolution and reconcile the instincts that compete within us," he said. "Now is the time for this great House of Commons to come together and bring people together."

Read more: The new Brexit deal: Will it pass? 

Opposition Labour leader Jeremy Corbyn urged MPs not to back the deal, saying it would put jobs, the environment and the UK health service at risk. 

Meanwhile, tens of thousands of anti-Brexit protesters were gathering in central London to demand another referendum.

Saturday's parliamentary session is Johnson's last chance to secure an orderly Brexit. If he fails, the prime minister must, by law, request a delay. Johnson has repeatedly said the UK would still depart from the EU on October 31 with or without a deal.

"There have been any number of false dawns. Deadlines for our departure have come and gone ... Today can be the day we get Brexit done," Johnson wrote in The Sun, a British tabloid newspaper on Saturday. "In less than two weeks, on October 31, we would be out of the EU."

In the case of a no-deal Brexit, the UK risks possible damage to global economic growth, violence in Northern Ireland and political unrest in Europe. 

The meeting is the first weekend sitting of the British Parliament since the invasion of the Falklands in 1982.

Read more: Opinion: Brexit once again, with feeling

Letwin amendment

A debate in parliament was to be followed by votes on amendments and then Johnson's Brexit deal — if all goes according to his government's plan.

But one of those amendments could lead to the vote on the Brexit deal being pushed to a later date, ultimately forcing Johnson to ask the EU for a Brexit extension until January 31, 2020.    

The proposal, put forward by MP Oliver Letwin, defers the decision on whether to approve any deal until all the legislation needed to implement the terms of the deal has been passed — a process that could take several days, if not weeks.

Johnson's spokesman said Saturday the government's focus is on "making sure that the Letwin amendment isn't passed." MPs who back the measure say it will prevent the UK from crashing out of the EU without a deal on October 31, and allow more time to study the divorce terms.  

Read more: What's new in this Brexit deal?

Letwin has said his amendment, which has cross-party support, is intended to be an "insurance policy" in case "something goes wrong during the passage of the implementing legislation."

The Labour party has said it will back the amendment.

Super Saturday

Johnson, whose party does not enjoy a majority in the divided parliament, must win the approval of 320 lawmakers across both his own Conservative Party and the opposition Labour Party to successfully ground his Brexit deal in British legislature.

His new Brexit deal already faced opposition from Northern Ireland on Thursday.

If he wins the vote, Johnson will have successfully fulfilled his campaign pledge to deliver Brexit to the British people. If he fails, Brexit will likely be delayed for a fourth time and the deal must extend beyond the October 31 deadline.

Saturday's vote comes more than three years after UK citizens voted 52% to 48% to be the first sovereign country to leave the European Union.

mvb/sms (Reuters, dpa)

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