Clark has reportedly been mulling over a tilt at the top job but announced on Monday she would not run, instead urging the eventual winner to push for the selection of a more diverse range of candidates.

Her decision could be good news for newly-returned MSP Monica Lennon, who will head back to Holyrood in September after Sarwar vacated his seat upon being sent to the House of Lords.

Ahead of nominations opening on Monday, Lennon claimed the contest “reeks of misogyny and male entitlement” amid reports she did not yet have the necessary support to make it onto the ballot.

The Glasgow MSP, along with competitors Michael Marra and Joe Fagan, is required to get the backing of three of the party’s MPs and three MSPs to continue in the process.

Announcing her decision, Clark – who is understood to be worried about a “continuity candidate” winning – said: “We need to remember that we are a socialist party. We were created by the trade unions and others to be a voice for working-class people.”

She added: “This leadership contest is the opportunity to discuss how we turn this idea of a Parliament which delivers for working-class communities and regenerates our local economies into a reality.

“That means selecting more candidates from working-class communities and more women, as well as a much better representation of those from ethnic minority backgrounds.”

She also urged the eventual winner to push for key public services – such as social care and buses – to be brought into public ownership.

Party members and affiliated supporters are due to vote between August 28 and September 17.

The new Scottish Labour leader will be announced on September 19.

Before nominations opened, Marra said: “I’m running in this contest to unite and renew the Scottish Labour Party.

“Having spoken to and listened to colleagues across our movement, it is clear we have to rediscover our sense of moral purpose.

“To offer hope again and to spell out what Scottish Labour is for, not simply who and what we are against.

“That means drawing on the strength and talent of all of our movement.”

Fagan, who was previously the leader of South Lanarkshire Council, had earlier welcomed the support of MSP Paul Sweeney.

He said: “Paul understands that Scottish Labour cannot be a party of the Holyrood bubble.

“We have to bring our party back to the service of our movement and the people of Scotland.”

All three leadership contenders have also written for the Scotland on Sunday newspaper, setting out their pitches to party members.

On Sunday, Lennon posted on social media saying there should be no “coronation or a contest without women”.

She said: “This ‘contest’ reeks of misogyny and male entitlement.  That’s the road to nowhere.  We’re not here for a social life or to win an occasional debate.

“We were created to win power for working-class communities and lead in government, not from the gutter.”