- Video report by ITV News Political Correspondent Libby Wiener
Boris Johnson's Brexit vision faces a roadblock when it goes before MPs on Saturday, with an amendment aimed at ensuring the UK does not leave the EU without a deal threatening to derail his plans.
Dubbed "Super Saturday", MPs are preparing to sit in the Commons on a weekend for the first time since the Falklands War in 1982.
Parliament will debate and then vote on the prime minister's newly acquired deal, which he hopes will help the nation "heave a great sigh of relief" should it be passed.
But not all in Westminster trust Number 10 to fulfill its Brexit pledges.
MPs from both sides of the House believe Mr Johnson may get his deal signed off by the Commons but that subsequent legislation - on such things as workers' rights or the environment - may not be agreed, meaning the UK could still leave without a deal at the end of the month.
Cue an amendment by Sir Oliver Letwin which could derail Mr Johnson's best made plans.
The West Dorset and former Conservative MP, who was one of the 21 MPs booted from the party after rebelling against the government, has been the mastermind behind several parliamentary moves to coerce the Downing Street's actions around the UK's divorce from Brussels.
That's if the Speaker of the House, John Bercow, agrees for it to be debated and voted on in the Commons.
- ITV News Political Editor Robert Peston believes the Letwin amendment will be passed - narrowly
While Mr Johnson has promised "the highest protections" and "the highest standards" after Brexit, Sir Oliver's amendment seeks to give MPs a safety net should he not do so.
In the event that parliament passes the new deal, the Benn Act - which forces the PM to write to Brussels asking for an extension if legislation isn't sorted in time for a Halloween Brexit - is effectively wiped out.
That would mean should MPs fail to pass bills put forward by the government ahead of the end of the month, the country would leave the European Union with no-deal.
- ITV News Europe Editor James Mates says European capitals are desperate to get the deal done
The Letwin Amendment means the PM would be obliged to write to the EU at 11pm on Saturday night asking for an extension to the UK's EU membership.
Whilst some will be grateful for the safety net, it will infuriate others - potentially kicking the Brexit can further down the road and well into the new year.
Will the deal pass through parliament?
Numbers are tight, it could go either way.
The prime minister doesn't enjoy a majority in the Commons - instead he
He needs 320 MPs to vote for his deal.
Assuming all Tory MPs vote with the government, it already has 259 votes already in the bag. That means he needs to pick up support from across the House.
If eurosceptic Tories in the ERG group support the plan, as well as 18 of the 21 Tory rebels booted from the party, that brings him to 305 ayes.
The DUP has 10 MPs - but is now almost certain to vote against the deal. That leaves the PM looking for support from Labour rebels to get him over the line.
- ITV News Political Carl Dinnen looks at how Boris Johnson's effort differs from Theresa May's deal