Boris Johnson is facing calls to sack the Welsh secretary, Alun Cairns, and remove him as a Tory candidate after he was accused of “brazenly lying” about his knowledge of an allegation that a former aide had sabotaged a rape trial.

Last week, the Welsh Conservatives claimed Cairns was unaware of Ross England’s alleged role in the trial before it had become public only days earlier.

But, on Tuesday, BBC Wales published an email it said was sent to Cairns by his special adviser more than a year earlier, in which the latter wrote: “I have spoken to Ross and he is confident no action will be taken by the court.” The Guardian has not been able to independently verify the report.

The email was sent on 2 August 2018, the BBC reported. Four months later, England was selected as the Welsh Conservatives’ candidate for the Vale of Glamorgan for the 2021 Welsh assembly election. He was later suspended after being accused by a judge of deliberately sabotaging the rape trial.

After he made claims about the victim’s sexual history while giving evidence, the judge, Stephen Hopkins, told England: “You have managed, singlehandedly, and I have no doubt it was deliberate on your part, to sabotage this trial.

“Mr England, as far as I am concerned, this matter so far as you are concerned isn’t ended. I shall be writing personal letters to people who are politically close to you and I hope they take appropriate action. Get out of my court.”

The defendant, James Hackett, was later convicted of rape at a retrial.

On Tuesday, the shadow Welsh secretary, Christina Rees, said: “There are no two ways about this: Alun Cairns has been caught brazenly lying about how much he knew of Ross England’s appalling behaviour at a rape trial.

“The secretary of state, special advisers and senior Welsh Tory officials knew what had happened. They chose to select Ross England anyway – and to mislead and obfuscate when found out. The decision is an error of judgment – the cover-up is unforgivable. Alun Cairns should go, as secretary of state and as a candidate.”

Plaid Cymru’s Westminster leader, Liz Saville Roberts, also called for Cairns to be removed as Welsh secretary and withdrawn as a general election candidate. In a letter to the prime minister, she said: “The revelation that Alun Cairns knew about Ross England’s role in this case before endorsing him as a candidate shows that Mr Cairns is unfit to hold public office.

“At worst, Mr Cairns is complicit in the attempted cover-up of his former staff member’s actions which collapsed a rape trial. At best, he has displayed gross incompetence in judgment, dishonesty and a lack of leadership.”

Liz Saville Roberts (@LSRPlaid) Letter to PM Johnson: remove Alun Cairns from your cabinet and prevent his standing as candidate.



The rape victim - a former Tory staffer - has called for Cairns’s resignation.



Surely her pleas come before this failure of a cynical, old-boys-network cover-up? pic.twitter.com/yfDXNQjCcf

A spokeswoman for the Welsh Conservatives said: “There is no new information from this leaked document confirming an informal conversation which took place a considerable time after the trial collapsed and is consistent with statements made.

“The full details of this case are still not known and we have taken action in writing to the court. All forthcoming information will be taken into account as the party conducts a thorough investigation.”