A confidante of the late Donald Dewar has said Labour leadership hopeful Andy Kerr is unlikely to win the contest because of past disloyalty to colleagues.

Sam Galbraith, the former education minister, said there was a "black spot" over Kerr because he used to act as a "mouthpiece" for former first minister Jack McConnell.

He also said the shift to the left by Cathy Jamieson, another leadership contender, would result in her not being trusted by the party.

Kerr, Jamieson and East Lothian MSP Iain Gray are in the race to succeed Wendy Alexander as Labour's Holyrood leader, with a winner to be announced next month.

Galbraith, a cabinet minister in Dewar's administration known for his trenchant views, is backing Gray, and has firm opinions on all three candidates.

On Kerr, who as a backbencher in the Dewar regime was aligned closely with the leadership ambitions of McConnell, the former MSP said: "It will be forever held against Andy that he was Jack McConnell's mouthpiece. It was not a good move for him. He thought it was subtle, but it was blatant."

He said Kerr was a "man of some ability", but added: "It's like all these people who brief off the record, they think nobody knows it's them, but of course everybody knows because your friends in the press tell you."

The elder statesman said he thought it unlikely that Kerr would attract enough support to win: "I think Andy has a good future in the Labour Party, but there is still that black spot over him, that disloyalty. Once you have done these things once, nobody ever forgets."

Galbraith also criticised Jamieson, whom he believes has moved to the left during the contest: "Cathy was old Labour until she became a minister, when she became solidly new Labour. She's trying to revert to win votes, but she shouldn't because people who swing about like that don't get trusted."

He said of his support for Gray: "I think he's the best candidate. He's got the most solid position."

In a dig at Kerr, he said of Gray's abilities: "He's generally liked, which is always a help. He's also strong and got the correct policies."

Gray, according to several senior Labour sources, is ahead in the leadership race, while Kerr is thought to be performing well amongst party members.

Ballot papers for the contest have been sent out and the winner will be declared on September 13.

Kerr said: "Sam is mistaken in his comments. I've always been a loyal member of the Labour Party and I'm disappointed to have lost this individual member's vote."

A spokesman for Jamieson said: "Sam's entitled to his view. He's very forthright, but I'm not going to get into a spat."