Chief Whip Julian Smith and Tory chairman Brandon Lewis are under intense pressure to quit in a row over claims they adopted murky tactics during a crucial Brexit vote.
A Tory former minister joined Labour in calling for the resignations after it emerged the failure to keep to a voting pact was actually planned rather than an “error” as claimed.
And backbench Tory MP Heidi Allen said she had challenged Mr Smith about the incident and he was “unable to confirm” that he did not order the breaking of the pact.
Theresa May is facing calls to apologise for misleading MPs about what happened and told she must sack the two men if they did not go voluntarily.
The decision to deploy dark arts tactics during the knife-edge vote on Tuesday drew criticism from across the political divide.
Labour Party chairman Ian Lavery said: “The Tories’ story is changing by the minute as they desperately scramble to cover up their appalling actions.
“This Government is rotten to its core. Julian Smith and Brandon Lewis must now resign or be sacked, and Theresa May must apologise for misleading the House.”
Tory former minister Anna Soubry, a leading Remainer, said: “If true this is appalling and those responsible must resign. If we cannot behave with honour we are nothing.”
Conservative Brexiteer Peter Bone said he was “very concerned” to hear that a pairing had been broken.
Ms Allen said: “Despite repeatedly asking Julian Smith direct questions this afternoon, he has been unable to confirm to me that he did not give instructions to break pairs.
“Therefore I can only conclude MPs were told to break pairs on Tuesday.
“When I became an MP in 2015, I was determined to challenge stereotypes about politicians.
“I refuse to be tarnished by this behaviour so will not stand by and say nothing. Integrity and honesty are fundamental to our democracy. Anything less is unacceptable.”
Mr Lewis had been “paired” with Liberal Democrat deputy leader Jo Swinson, meaning neither would walk through the voting lobbies.
Mrs May told MPs the breaking of the pair was “done in error”.
But The Times reported that the party chairman and two other Tory MPs were told by Mr Smith that they should go ahead and vote despite being paired.
It said the two unnamed MPs both sought further advice and ignored the instruction.
The Conservative Party did not deny the allegations.
Ms Swinson said: “Well … This reflects pretty badly on those peddling the ‘honest mistake’ nonsense.
“To be fair, hats off to the two MPs who told their chief whip to take a running jump when he asked them to break a pairing just because the govt might lose.”
The actions did not change the result of the votes but the Government only narrowly scraped through.
Downing Street said Mrs May still had full confidence in Mr Smith.
Asked if the PM stood by her comments that the pact had been broken in error, a spokeswoman replied: “Yes, absolutely.”
Lib Dems called for the Chief Whip to make a statement to the House.
SNP Westminster leader Pete Wishart asked in the Commons for a full inquiry into the breakdown of pairing.
A Conservative spokesman said: “We have apologised for the fact that a pregnancy pairing arrangement was broken in error this week.
“No other pairs offered on the Trade Bill on Tuesday were broken.”
Commons Leader Andrea Leadsom said there would now be a debate on proxy voting, which would end the need for pairing, in September.
Over the course of the Parliament 66 pairing arrangements have been broken, with 52 of those at the hands of the Opposition, according to Tory sources.
Conservative whips did consider breaking short-term pairing arrangements – those for MPs who need to be away for another engagement, sources acknowledged.
But they said long-term pairing for maternity and health issues was something they stick to and insisted breaking the agreement with Ms Swinson had been a mistake.
It is understood that is the error the PM referred to when she spoke to MPs about the issue.