More than 60 MPs are set to fight an amended motion backing the government's timetable for Brexit later today.
The Lib Dems and the Scottish National Party have both said they will reject the motion in the House of Commons, which calls on the government to publish a Brexit plan ahead of the triggering of Article 50.
Crucially, Theresa May's officials issued an amendment to the motion yesterday, agreeing to reveal some information in exchange for the support of MPs in her plan to begin Brexit talks by the end of March.
But two of parliament's most Europhile parties, the SNP and Lib Dems, who collectively share a total of 63 MPs, have both rejected it.
The SNP is pushing for a clause to make clear Parliament's support for handing the Scottish government a formal role in the UK's Brexit negotiations, while the Lib Dems say the government must commit to a second referendum on the terms of Brexit.
And Lib Dem leader Tim Farron also said the government had provided no commitment to significant disclosure.
"An amended motion would fail to include any meaningful commitment from the Conservative Brexit government to produce the equivalent of a White or Green Paper setting out its position on such fundamental questions as to whether it wants Britain to remain in the Single Market," Farron said.
The debate will begin at roughly 12.45 today.