Dozens of hours of parliamentary guerrilla warfare are beginning as MPs prepare to debate the small print of the Government's Brexit legislation for eight days and nights between now and Christmas.
The committee stage of the EU (Withdrawal) Bill gets under way with Labour and rebel Conservative MPs attempting to inflict a series of damaging defeats on Theresa May and Brexit Secretary David Davis.
Only hours before the start of the debates, Mr Davis attempted to buy off a rebellion by pro-Remain Tories by promising a further piece of legislation giving MPs a "take it or leave it" vote on the final Brexit deal.
But his concession has failed to win over many MPs, who claim the vote should be held earlier and that it fails to solve the problem of so-called "Henry VIII powers", which give ministers scope to change laws after Brexit.
The committee stage begins with four hours of debate on the precise timing of when the UK leaves the EU, followed by another four hours of debate on calls for a bigger Brexit role for the Scottish and Welsh governments.
Labour Eurosceptic Frank Field is proposing a new clause saying the UK should cease to belong to the EU on 30 March 2019, a day after the Government's proposed deadline – announced last week – of 11pm on 29 March 2019.
Day two of the debate, on Wednesday, will see eight hours of debate on Labour demands for guarantees on workers' rights and environmental protection, though the party's high command is not confident of defeating the Government.
The first real threat of a Government defeat is expected next Tuesday, on the eve of the Budget, when MPs debate the "Henry VIII" powers and Mr Grieve, Mr Clarke and other Remainers are confident of a rebellion by up to 10 Tory MPs.
More than 400 amendments and new clauses to the Bill have been tabled, prompting the Government to delay the committee stage for weeks while ministers tried to find ways to head off Tory rebellions and potential defeats.
Speaking in Birmingham ahead of the committee stage, Jeremy Corbyn said: "Today the misnamed EU Withdrawal Bill came back to Parliament. It is in fact an undemocratic government power grab.
"Its return follows weeks of damaging delay. That has only added to the sense of chaotic dithering around the Conservatives' entire approach to Brexit.
"Nearly 17 months since Britain voted to leave the EU, we are still none the wiser as to what our future relationship with our biggest trading partners is going to look like.
"The Government can't give a lead because the Cabinet is split down the middle. Ministers spend more time negotiating with each other than with the EU. That gives the whip hand to grandstanding EU politicians.
"One week the Home Secretary says a no-deal exit from the EU would be unthinkable. The next week the Brexit secretary insists no deal must be an option."
With MPs challenging the Government on what happens if Mr Davis and the EU's Michel Barnier fail to reach a Brexit deal, the SNP's spokesman Stephen Gethins said: "A no-deal scenario must be taken off the table.
"The SNP have put forward an amendment to the EU Withdrawal Bill – a reset amendment – which would ensure that the UK can remain in the EU. We must ensure the best possible outcome.
"We should also not be tied to an arbitrary date for leaving the EU if a deal is not done. The Prime Minister needs to act in the best interests of the country."
While the Commons debates get under way, a group of pro-Brexit economists is claiming the Government can look forward to a £135bin windfall after the UK leaves the European Union.
In a new report, the Economists for Free Trade (EFT) group says Brexit will be "overwhelmingly positive" for the British economy, provided the Government adopts the right policies.
The EFT – headed by right-wing economist Professor Patrick Minford – says the priority for the Government should be to bring down trade barriers with the rest of the world while reducing regulation and taxation on firms and individuals.
Speaking at the report's launch, the pro-Brexit Conservative MP Jacob Rees-Mogg will say: "This is a free trade approach that focuses on consumers, not producers; one that will generate gains to consumers seven times the cost to producers.
"This is a free trade approach to Government which believes Britain's greatest days lie before it and not behind it."