David Davis today issues a last-minute warning to Tory rebels not to sabotage the Article 50 Bill amid fears any change will see Brexit back in the courts.
The Brexit Secretary says that putting promises over leaving the EU into law creates a "greater risk of legal action". And he warns that Prime Minister Theresa May would be negotiating with "one hand tied behind her back" if MPs approve two amendments to the existing Bill proposed by the House of Lords.
Davis also says that protecting the rights of EU citizens in the UK without reciprocal promises from Brussels would plunge more than a million Britons resident abroad into "uncertainty".
He writes: "There will be many opportunities for Parliament to debate the ins and outs of our negotiation of a new partnership with the EU, and influence the outcome. But attaching conditions to a Bill that simply allows the Prime Minister to start the process of implementing the referendum result is emphatically not the way to do it."
The shot across the rebels' bows comes ahead of a historic week in Parliament that is expected to see Britain's withdrawal from the EU approved by both MPs and peers.
Theresa May is preparing to formally start talks as early as tomorrow in a landmark moment, dubbed "independence day" by Brexit supporters.
However, before that can begin, MPs must vote on whether to make the two amendments to the Bill. The first demands that proposals to protect the rights of all EU citizens currently in Britain be published within three months; the second gives Parliament a "meaningful vote" on the final Brexit deal - effectively a veto.
The Government opposes both, but pro-EU Tories are rallying privately behind both changes. Only around 30 Tory rebels would be needed to back the changes for them to pass, given that opposition parties support them.
Tory whips are so concerned that Cabinet ministers have been forced to cancel foreign trips to make sure they attend the vote while the whips have refused to approve any "slips", which give MPs permission to be away. "It is impossible to get away at the moment. If this carries on it increases the incentive to go for an early election," one MP said.
Davis warns off any Conservative MPs who might consider backing either amendment. "At 137 words, the Bill which enables the Prime Minister to notify of our withdrawal from the European Union is one of the shortest on record," he says.
"Yet it has generated many hours of debate in Parliament. That's to be expected, and a good thing... however, by a majority of four to one, the elected House of Commons accepted the simple, straightforward and clear aim of the Bill. That is to allow the Prime Minister to implement the outcome of the EU referendum, while respecting the judgment of the Supreme Court that this should be authorised by legislation. No more, no less."
Davis says making promises to EU citizens in the UK without achieving a reciprocal promise for Britons abroad "risks exposing UK citizens in the EU to a long period of uncertainty".
He says calls for a "meaningful vote" on the deal is a "veto" and he notes that the PM has already committed to a vote. He also raises the spectre of Brexit ending in the courts if the changes are passed.
Pro-EU Tories have rejected claims that the amendments would undermine May's negotiating stance as "absolute rubbish".
One said: "What we will not have is this ideological claptrap for hard Brexiters who want take us out of the EU without a deal."
MPs need to approve the Article 50 Bill today. It then has to clear the Lords later this morning. The Prime Minister would then be free to start Brexit tomorrow night NZT. No 10 is preparing a letter to be sent to Brussels and a speech.