UNDER pressure Theresa May is standing by her Chief Whip and the Tory Party Chairman after calls were made for them to go in an escalating Westminster row over claims they adopted underhand tactics during a crucial Brexit vote.

Labour claimed the controversy proved the Conservative Government was “rotten to the core” after it was suggested the failure to keep to a voting pact with opposition parties was planned rather than an unintended "error" as previously claimed.

The Prime Minister herself is now facing calls to apologise for misleading MPs about what happened and told she must sack the two men if they did not go voluntarily.

Yet No 10 has made clear Mrs May continues to have full confidence in Scots-born Chief Whip Julian Smith, saying she still believed that the breaking of the pairing arrangement between Brandon Lewis, the party Chairman, and Jo Swinson, the Deputy Liberal Democrat leader, who is on maternity, was an honest mistake.

Criticism over the claims that Mr Smith had deployed dark political arts during Tuesday’s knife-edge vote on the customs union, which the Government won by just six votes, came from across the political divide.

The SNP’s Pete Wishart demanded a parliamentary inquiry into the whole saga.

It is thought as many as eight Conservative MPs were paired with their opposition counterparts, so that they would cancel each other out on the key customs union vote.

But it is suggested such was the concern the Government would lose the vote, Mr Smith ordered five colleagues to break their pairing arrangements; only Mr Lewis did so, the others refused.

One of the Tory MPs who was paired told The Sun: “Julian told me I was needed and told me to come in and vote. Of course, he knew I was paired. I didn’t vote and honoured my pair and he demanded to know why not afterwards. It then appears Julian told the Prime Minister it was all an innocent mistake.”

The irate backbencher added: “What happened was unacceptable. We cannot behave like this”.

Tory HQ has not yet denied the allegations.

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."

Andrea Leadsom, the Commons Leader, said there would now be a debate in September on proxy voting, which would end the need for pairing.

But Ian Lavery, the Labour Chairman, was incensed, saying: "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."

Ms Swinson, who represents East Dunbartonshire, tweeted: "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."

Tory MP Anna Soubry, a leading Remainer, also expressed concern, saying: "If true, this is appalling and those responsible must resign. If we cannot behave with honour we are nothing."

Her Conservative colleague Heidi Allen said: "No matter how tough the going gets, principle, integrity and standards matter. Without those, what's left?"

Alistair Carmichael, the Lib Dems’ Chief Whip, who secured an urgent question on the matter on Wednesday, later told MPs that after meeting Mr Smith he still did not understand how the error had occurred.

Speaking at Commons Business Questions, the MP for Orkney and Shetland, said: "I have somewhat of a novel request for the Leader of the House and that is that the Government Chief Whip should come to the despatch box and make a statement himself.

"Now I've been here long enough to know the conventions, I know the convention is that the Government Chief Whip does not speak in the House, but conventions are exactly that, ours is a system of checks and balances, if you take out a check then you have to adjust a balance," he added.

Ms Leadsom did not respond directly to the request but again apologised for the "error".