The government said it would publish its Withdrawal Agreement Implementation Bill (WAIB) this afternoon, with a vote expected tomorrow. The UK is due to leave the EU in ten days.
Opposition MPs have threatened to attach amendments calling for the whole of the UK to stay within the EU’s customs regime, as well as ordering that it be subject to a second referendum on Brexit.
The Prime Minister’s official spokesman said that any changes to the legislation that would implement the Brexit deal that “steps too far away from what was agreed in the Withdrawal Agreement and the political declaration, that does bring into question ratification.”
A previous vote on staying in the customs union was defeated by just three votes - although three Tory MPs who backed it are now government ministers, and another supporter, the Scottish Tory Paul Masterton, said he would not do so again.
“This argument is for future relationship discussions, not a tool to be used to try and derail the deal being agreed,” Mr Masterton posted on twitter.
The Downing Street spokesman also said that any attempts to amend a motion for a ‘meaningful vote’ on Brexit which will be brought this afternoon would see ministers call it off.
"The meaningful vote will go ahead if the Speaker allows it and if not, and amendments are selected which would render the vote pointless, there's no point having a meaningless vote,” the spokesman said. “The government would pull the motion.”
However, Commons Speaker John Bercow is expected to rule out another vote later on Monday in any event, because MPs passed an amendment on Saturday from the former Tory MP Oliver Letwin, preventing ratification of the Brexit deal until the WAIB is passed.