The prime minister has refused to be drawn on whether he should sack Tory assembly candidate Ross England.
Mr England was selected by the Conservatives eight months after he was accused by a crown court judge of deliberately sabotaging a rape trial which collapsed.
Boris Johnson said it would be “inappropriate for me to comment on ongoing legal proceedings”.
Legal proceedings have concluded in the case.
The defendant, James Hackett, was later convicted following a retrial. Cardiff Central MP Jo Stevens said Mr England’s selection was “unbelievable”.
Ross England was giving evidence in a rape trial in April 2018 when he made claims about the victim’s sexual history, which the complainant denies.
In December 2018 he was selected for the Conservatives in the Vale of Glamorgan seat.
Mr England has worked for Alun Cairns, the Welsh Secretary and Conservative Vale of Glamorgan MP.
At prime minister’s questions in the Commons Ms Stevens said: “Yesterday it was reported that a former staff member of the secretary of state for Wales, Ross England, had in the words of a trial judge single-handedly and deliberately sabotaged a rape trial by referring to the victim’s sexual history against the judge’s instructions.
“The trial had to be stopped, and started again from scratch and the defendant was convicted.
“Unbelievably the party then selected Mr England as a Welsh Assembly candidate with the Secretary of State’s endorsement. Is the prime minister going to sack Mr England?”
In response, Mr Johnson said: “It would be inappropriate for me to comment on ongoing legal proceedings.”
The Conservative party has been repeatedly asked for comment by BBC Wales.
None has yet been provided.
Earlier on Wednesday former Plaid Cymru leader Leanne Wood said Mr England should be deselected.
“This whole case and the actions of the Tories in this absolutely stinks,” she said.
“Ross England should be sacked as a candidate now and it would do no harm for the Tories to understand our strength of feeling.”
Giving evidence in the April 2018 trial Mr England made claims that he had had a casual sexual relationship with the complainant, which she denies.
The trial judge in the case, Stephen John Hopkins QC, had earlier made clear that evidence of the sexual history of the victim was inadmissible.
He went on to say to Mr England: “Why did you say that? Are you completely stupid?”
Mr England said that he thought the question was about his relationship with the woman. Replying, His Honour Judge Hopkins said it was not: “It was quite clear what the question was.”
The judge then said: “You have managed singlehanded, and I have no doubt it was deliberate on your part, to sabotage this trial”.
Analysis by Felicity Evans, BBC Wales political editor
The timing of this could not be worse for the Welsh Conservatives.
It raises serious questions about the rigour of their candidate selection procedures at the very time they’re about to ask you to vote for general election candidates they have selected.
It also risks their campaign being overshadowed by a row over whether Ross England – a candidate for a completely different election – is fit to have the endorsement of the party.
Mr England denies wrongdoing. Nevertheless a Crown Court Judge threw him out of court for “sabotaging” rape trial with very serious consequences.
The trail had to be abandoned, costing taxpayers tens of thousands of pounds. A rape victim endured the added trauma of having to go through an extra trial – the delivery of justice to a rapist was delayed.
The party may have an explanation about why it selected a candidate who, for whatever reason, did not do what hundreds of witnesses in criminal trials do every working day – giving evidence without derailing the entire process. So far that explanation has not been forthcoming.