A Conservative ex-minister has defended the “integrity” of Sue Gray following the appointment of the partygate investigator to Labour’s staff.

It comes as Labour insisted that all civil service rules will be followed amid outcry from Conservative MPs.

The planned move of Ms Gray, who received national prominence for her role investigating lockdown-breaking parties in Downing Street, to a top role on Sir Keir Starmer’s staff surprised many in Westminster.

It has been leapt on by Boris Johnson and allies as part of attempts to discredit the Privileges Committee inquiry into whether he lied to the House of Commons over lockdown breaches.

Ms Gray received backing from a former Conservative Cabinet Office minister on Saturday as Francis Maude said he had never the “slightest reason to question either her integrity or her political impartiality”.

In a letter to the Times, the Conservative peer said that Ms Gray, who worked as his principal private secretary for a period of time, was not the first civil servant to move to a political role and would not be the last.

“We should be as relaxed about this as we should be about people who have had previous political involvement coming into the civil service,” Lord Maude said.

“Regardless of any political background or leaning, civil servants must of course comply with the civil service obligation of impartiality while they are in post.

“Our civil servants should have brains, knowledge, judgment and strength of character to give robust advice to ministers. Gray has all these qualities in abundance; Starmer is fortunate to have secured her services.”

Sir Keir has so far dodged questions about when conversations began with Ms Gray, who is expected to await the decision of the Advisory Committee on Business Appointments (Acoba) before starting the role.

Keir Starmer visit to Derry
Labour leader Sir Keir Starmer faced questions about the appointment while on a visit to Northern Ireland on Friday (Brian Lawless/PA)

Parliament’s anti-corruption watchdog can advise waiting periods before civil servants take on other jobs and the Prime Minister ultimately makes the final decision.

On Saturday, Labour chairwoman Anneliese Dodds rejected the suggestion that the move was a “distraction” from the Privileges Committee inquiry as she insisted that all necessary procedures would be followed.

“Sue Gray is a person of enormous integrity. Someone who served in the civil service under ministers of a number of parties actually, someone who’s always served with that integrity”, she told Sky News.

“I’m really delighted she’s joining the Labour team at that point where we’re readying ourselves for government if the British public backs us at the next general election.

“What’s important to us as Labour, as ever, is that we see the same rules and approaches being applied to this, as she would see with any other appointment. That’s why the civil service procedures on confidentiality will be followed.

“It’s why the civil service watchdog Acoba will need to look at this, just as it would with any other appointment, and it’s quite right those procedures will be followed. They will be for Sue Gray, just as they would be for any other senior civil servant.”

But former Conservative leader Sir Iain Duncan Smith questioned the wisdom of the move.

“There needs to be a much clearer sense for civil servants that once you go into politics it’s a different game altogether,” he told Times Radio.

“You have to therefore have a big break between taking off from your job and going in, particularly if you’re senior.”