Carrie Johnson is being targeted in a “deeply unsavoury” political attack over allegations Boris Johnson tried to get his then girlfriend a senior job at the Foreign Office in 2018, the deputy prime minister has claimed.

Dominic Raab, the justice secretary and Johnson’s second in command, said Carrie Johnson was being criticised as part of a “feeding frenzy” against the prime minister.

Speaking to Sky News, he batted away calls for an investigation into the allegations, which Labour’s Chris Bryant, the standards committee chair, said appeared to be a case of corruption.

The claims were first made in a book by the Tory peer Lord Ashcroft and came into the spotlight again after the Times withdrew an article going into more detail about how Johnson had tried to get his now wife a £100,000 post at the Foreign Office where he was foreign secretary in 2018. Johnson was still married to Marina Wheeler at the time.

No 10 later admitted having intervened to stop the article from running, despite the journalist who wrote the story, Simon Walters, standing by it. The Guardian has also been told by sources that the story is true, with one saying evidence exists that Johnson tried to get his then girlfriend a job.

Asked whether there should be an investigation, Raab said: “These attacks on Carrie are deeply unsavoury. The allegations have been roundly responded to as flawed. Those are matters for No 10 and Carrie. But frankly, what I think you’re seeing is a political attack on Carrie as a way of getting to the prime minister and that’s out of order.”

He added that there was a “feeding frenzy on anything thrown out there as a claim against the prime minister and his wife and I think that’s wrong”.

The prime minister’s spokesperson has previously said they were unable to comment on Johnson’s activities before he became prime minister, but that “others have made clear this story is untrue”.

The former Foreign Office minister Alan Duncan said he was told in 2018 that Carrie Johnson was a rising star running communications in Conservative campaign headquarters (CCHQ) and was being lined up for a special adviser role in the department.

“For someone slightly unproven who knew nothing about foreign affairs to come straight out of CCHQ and into the Foreign Office was rather noticeable,” Duncan told the Guardian.

He said he had had no idea of the nature of her relationship with Johnson and the suggestion about her rapid promotion had only been cursorily mentioned, adding: “Had it got closer to the goal, people who knew more would have revealed more and then the bubble would have burst.”

Duncan said the suggestion Johnson had tried to get his girlfriend a £100,000-a-year taxpayer-funded job was the latest evidence of his “dripping ceiling theory”. “They put one bucket under one drip, another bucket under another drip – but at some point the whole ceiling will come falling down.”

On Tuesday Bryant called for the “paper trail” surrounding the issue to be published. “It is manifestly corrupt to appoint your lover as a spad,” he said in a tweet.

Bryant was urged by the Liberal Democrats’ chief whip, Wendy Chamberlain, to investigate the allegations, given the government’s ethics adviser role remains unfilled after the resignation of Christopher Geidt last week.

After Lord Geidt said the claims “could be ripe for investigation”, Chamberlain said there was a “significant risk that no such inquiry will follow” because No 10 had not committed to replacing the adviser while it reviewed the role.

Chamberlain said: “If found to be true, these allegations would amount to a serious breach of standards in public life and bring not just the office of prime minister into disrepute, but parliament and our politics as a whole.”

Given the standards committee can only conduct general inquires and those referred to it by the standards commissioner, Bryant would have to launch an investigation into “ministerial attempts to appoint people of interest” and then call Johnson as a witness to formally examine the case.

Carrie Johnson’s spokesperson has called the claims about her being offered a job in the Foreign Office “totally untrue”. On the subsequent claims about attempts to find her a job elsewhere, they said: “This is an old story, as untrue now as it was then.”