Keir Starmer has repeatedly refused to say when he first spoke to Sue Gray about becoming his new chief of staff.
A row has erupted over the move, with Boris Johnson and his supporters claiming it shows her report into partygate was a political stitch-up.
Gray, who has held a series of senior roles during decades in the civil service, will today set out the details surrounding her move to the Advisory Committee on Business Appointments watchdog.
Critics have called on Starmer and Gray to come clean on when they first discussed her appointment.
Asked on LBC this morning when he first approached her about it, the Labour leader repeatedly dodged the question.
He said he had first met her when he was Director of Public Prosecutions, but insisted he never had “a discussion with her during the entire time she was doing her [partygate] report” last year.
LBC presenter Nick Ferrari then said: “Might I ask when you first approached her to be your chief of staff?”
Starmer replied: “I’ve been on the lookout for a chief of staff for a little while now, I’m very clear what I wanted in that, and obviously Sue will set out that, but nothing improper at all.
“I’ve been on the lookout for a chief of staff, I’m really pleased that people of her calibre are interested in that.”
Ferrari then said: “Can I ask when you approached Sue?”
Starmer would only say he had been looking for someone to take on the job “for a number of weeks”, but would not go into specifics about when his discussions with Gray took place.
Asked again by Ferrari when he first approached her, Starmer said: “Well that’s going to be laid out by Sue, she’s got to do that as part of her leaving procedure, but there’s nothing improper at all.”
Ferrari then tried once again to get clarity from the Labour leader, who replied: “I’m not going to go through lock, stock and barrel, but there’s nothing improper - it’s not unusual.”