The cabinet secretary revealed that in 2020 he was prompted by a now-former member of the team at No 10 to ask the Royal Foundation about a position at the Earthshot Prize for Mr Johnson’s then-fiance.
But Mr Case – who has close links to the Duke of Cambridge, being his former private secretary – insisted that he had not endorsed her for any paid work.
He made the admissions in response to a letter from Labour deputy leader Angela Rayner, who demanded that he explain his involvement in Mr Johnson’s alleged efforts to secure top jobs for Ms Johnson – and specifically whether he had approached the Royal Foundation.
In response, Mr Case insisted there had been “no improper conduct”, saying: “I did not recommend Mrs Johnson for any role.
“In autumn 2020 a former member of the Number 10 team asked about opportunities for Mrs Johnson with environmental charities.
“I was happy to have an informal conversation with someone involved with the Earthshot Prize about what roles were available, as I would have done for anyone with relevant experience who was keen to get involved with charity work.”
It comes after the prime minister did not deny claims he had spoken to his aides about environmental roles for his wife in 2020, either for the Cop26 summit or with the royals.
It followed reports that, when Mr Johnson was foreign secretary in 2018, he attempted to hire Ms Johnson – who was then known by maiden name Symonds – as his £100,000-a-year chief of staff while he was still married to his second wife Marina Wheeler.
In her letter to Mr Case, Ms Rayner said that the potential conflicts of interests raise “serious questions about sleaze and corruption” in Downing Street.
She wrote: “There are now serious questions for the prime minister to answer about potential impropriety and conflict of interest in his lobbying for a high-paying tax-funded job for his partner in 2018, Carrie Johnson.
“Reports suggest that while foreign secretary, Mr Johnson tried to get Ms Johnson appointed to a Foreign Office job in 2018, and to other roles including one with a royal charity.”
Ms Rayner went on: “It is clearly inappropriate that the prime minister should be the judge and jury in his own case, so I ask you to confirm that this matter will be subject to an independent probe and fully investigated.”
She told Mr Case that she was writing to him “in the absence of an Independent Adviser of Ministers’ Interests” following the resignation of Mr Johnson’s independent ethics adviser Lord Geidt last week.
Ms Rayner said she was concerned about the “glaring hole in the standards system” left by his resignation.
She also wrote separately to Lord Evans, chairman of the Committee on Standards in Public Life, calling on him to “instigate an independent investigation into this matter”.
Downing Street said Mr Johnson had never recommended Ms Johnson for a government role, but stopped short of denying that he considered or discussed the move.
The prime minister’s official spokesperson this week said: “The prime minister has never recommended Mrs Johnson for a government role, or one as part of the Earthshot Prize.
“Beyond that I wouldn’t get into any conversations the prime minister may or may not have had in private.”