Liam Fox has warned Tory MPs they will undermine the prime minister’s negotiations with Brussels if they vote against the government on Thursday.

Brexiteer Tories are threatening to rebel in a key vote this evening over a motion they claim would commit Theresa May taking a no-deal Brexit off the table.

It asks MPs to reiterate their support for the approach agreed on January 29, when they backed an amendment authorising May to go back to Brussels to renegotiate the controversial Irish backstop.

But members of the backbench European Research Group (ERG) say it effectively endorses another amendment approved by MPs the same day, which rules out no-deal.

Speaking to BBC Radio 4′s Today programme, Fox pleaded with eurosceptic Tory MPs to put aside “ideological purity” and back the PM.

“What we say is looked at and listened to by those that we are negotiating with,” the international trade secretary said.

“They will be looking to see whether parliament is showing consistency because in the debates and the votes we had recently there were two things: one was that parliament said we don’t want there to be no-deal, and that we will sign up to the prime minister’s deal if we are able to get changes to the Irish backstop.

“Our European partners will be watching our debate and listening today to see if they get the impression that if they were to make those concessions, parliament would definitely deliver on that.”

Former Brexit minister Steve Baker, a leading figure in the ERG, said Conservatives should not be “associated with anything” that “seems to take no deal off the table”.

In a series of tweets in response to Fox’s interview, he said: “Ok: let’s send the right signals. (But is the EU really hanging on a non-binding motion at 5pm on a Thursday afternoon?)

He said: “Compromising no deal would be the daftest negotiating strategy and not in the national interest.”

Nicky Morgan, a pro-EU Tory, also hit out at the government. “If the government had taken the time to discuss the wording of the motion with MPs on all sides of the Brexit debate they’d have achieved a clear & united signal to the EU as Brady did on 29th January,” she tweeted.