Senior Tory Andrew Tyrie is demanding the government explains its hopes for the Single Market as part of a Brexit plan promised last year.
Last year, Prime Minister Theresa May committed to revealing more on the UK goals ahead of triggering Article 50, and now Tyrie, who chairs parliament's influential Treasury Committee, is demanding that any disclosure includes specific information.
In a speech tonight, Tyrie will call on May to state definitively whether or not the UK will retain memberships of the Single Market and the Customs Union.
Remaining within the latter would limit the UK's ability to secure trade deals with other nations after Brexit, but could improve trade to Europe.
Lastly, Tyrie demands clarity on transitional deals, and specifically whther EU treaties may continue to apply to the UK for an agreed period after negotiations have been completed.
Unlike an extension of negotiations, which would require unanimity among European member-states, this could be approved by a majority group.
“There is almost certainly a majority in the country – and a cross-party majority in Parliament – for a continued close economic and political relationship with the EU from outside,” Tyrie said.
“Safeguarding the economic wellbeing of the country probably requires a relationship that is considerably deeper than that provided for under WTO rules.
“Given the need to build a broad-based support for its position, at home and abroad, the sooner the Government can provide clarity, the better.”