MPs are scrambling to challenge Boris Johnson as he tries to rush his Brexit deal through Parliament in just three days.
Amendments to are piling up in Commons offices as MPs try to stop the PM ramming through his form of Brexit.
Major flashpoints will include a bid for a customs union and second referendum.
MPs also want to close a 'trapdoor to no-deal' at the end of 2020.
But he could abandon the Bill altogether if its bracing timetable for debate in the Commons gets voted down - or it gets amended out of shape.
Here are the main amendments we've spotted so far.
Customs union
This will be the biggest flashpoint of the debate.
Whether to scrap Boris Johnson's plan to quit the EU customs union, allowing the UK to strike its own trade deals, and instead have a customs union with the EU.
Labour wants a UK-EU customs union and is set to move or back an amendment.
One amendment is by ex-Tory Ken Clarke, and backed by Labour frontbencher Jack Dromey along with Labour MPs who back Boris Johnson's deal.
Crucially they call for a customs union not now, but some time down the line as part of the "future relationship".
The government would have to declare an "objective" to secure a "comprehensive and permanent customs union for the whole of the UK with the EU" before the end of the transition period.
Ministers would also have to update the Commons with progress reports every three months.
Second referendum
Labour is expected to back an amendment that would force Boris Johnson's deal to be put to the public in a second referendum.
An amendment had not formally been tabled at time of writing, but it would come to a dramatic vote at around 1am on Thursday.
Closing the no-deal 'trapdoor'
Labour has launched a bid to close the "trapdoor to no-deal" at the end of 2020.
Under the current Bill, there is no way for MPs to force the Prime Minister to ask for an extension to the transition period if there's still no trade agreement.
That could leave the UK crashing out with no deal on 1 January 2021 when the transition period ends.
Labour's plan would force every UK member of the 'Joint Committee' with the EU, which will negotiate any extension, to be appointed with the individual approval of Parliament.
That way Boris Johnson would be prevented from stuffing it with hardliners who don't want delay.
Delay Brexit until 2023
Tory MP Nick Boles tabled a bid to force the Prime Minister to seek a two-year extension to the transition period.
This would mean the new cut-off date for no deal if there is no trade agreement is 1 January 2023.
Give EU citizens a solid document guaranteeing they can stay
Several amendments would seek to secure EU citizens' right to stay in the UK - which is already guaranteed for those who arrive by 31 December 2020.
Two cross-party amendments by MPs - Lib Dems, SNP and Chris Leslie - would give EU citizens a residence document guaranteeing they can stay in the UK.
The idea is they will then have "proof" they can stay if there is a mix-up with the Home Office.
A third amendment by the same group of MPs guarantees that nothing in the Bill will stop EEA and Swiss nationals applying for settled status if they arrive before December 2020.
Transparency over EU courts in UK
Several amendments would force the UK government to make clear where it is still following or interacting with EU court rulings after exit day.
A Labour amendment would force the government to publish reports and make statements to Parliament on new issues which have been referred to the European Court of Justice.
And a bid by independents Anna Soubry and Chris Leslie would force ministers to publish a report spelling out how far the European Court of Justice affects us in the transition period.
This will be valuable transparency - but would also be highly embarrassing to the government which claimed we'd be free of EU courts.
Sovereign Parliament
A cheeky amendment from independents Anna Soubry and Chris Leslie would take aim at the Bill's bizarrely unnecessary claim that Parliament is sovereign.
They would simply add: "... and always has been sovereign."
