A Cabinet Office investigation into how much the former Wales secretary Alun Cairns knew about the role of an ex-aide in the collapse of a rape trial has been branded a “sham” by the female victim in the case.

The MP for the Vale of Glamorgan stepped down from the cabinet after it was revealed he had endorsed his former adviser as a Welsh assembly candidate even though he had been accused by a judge of sabotaging the trial.

Cairns claimed he was unaware of the role played by Ross England in the collapse of the court case until after the story broke in October. A Cabinet Office investigation found it was unlikely that he was not told about England’s role, but concluded that Cairns had not breached the ministerial code.

The rape victim, another former member of Cairns’s staff, told BBC Two’s Victoria Derbyshire programme on Thursday that she was not contacted by the investigation and claimed it had been merely a “deflection” until after the general election.

The woman, who cannot be named, said: “The whole thing feels like a sham. What kind of investigation doesn’t contact the person who is most affected? It makes me question how seriously it was actually carried out. This was a deflection until after the general election, not a genuine investigation into Alun’s truthfulness.”

She added: “I would’ve hoped Alun would’ve used the inquiry as an opportunity to be honest, and it’s regretful that he did not. Fortunately nobody with an ounce of common sense believes this ludicrous claim that he did not know.”

England’s friend James Hackett was found guilty of raping the woman at a house party and was jailed for five years. An initial trial collapsed after England broke a judge’s ruling by making claims in court about the woman’s sexual history. The judge said he was in no doubt that he had made the statements to sabotage the trial, though England said he had not known about the ruling.

The woman said Cairns had told her she was “being a bit harsh” in pursuing the rape complaint against Hackett and that she had been subjected to a “smear campaign” by individuals inside the Conservative party, who accused her of being a liar.

The Cabinet Office report by Sir Alex Allan, the prime minister’s independent adviser on ministers’ interests, concluded: “I find it unlikely that Mr Cairns would not have been told something about Mr England’s role when he was told about the collapse. But all those involved state that they had not informed Mr Cairns of Mr England’s role, and there is no direct evidence to contradict this. On that basis, I do not find that the evidence upholds the allegations of a breach of the ministerial code.”

Cairns was replaced as Wales secretary by Simon Hart following the Conservatives’ election victory in December.

The Conservative party in Wales said in a statement: “We are deeply sorry for the circumstances surrounding the collapse of the trial and the deep distress this must have caused the victim.”

Cairns told the BBC he was “extremely sorry for the trauma and suffering [the victim] has faced”. “I believed that my parliamentary staff and I had supported her in a caring and compassionate way throughout,” he said. “[The victim] recognised this in messages received at the time.”