A Scottish Labour MP has claimed his party's leadership election is being "rigged" by the country's largest trade union to help left-wing candidate Richard Leonard.

Ian Murray, who was the party's sole Scottish MP until June's snap election, made the accusation in a letter to the party's general secretary Brian Roy.

Unite has called the MP's comments a "baseless smear".

The private correspondence between the pair emerged after it was leaked to the Herald.

Murray, who supports Leonard's opponent Anas Sarwar, is unhappy after Unite sent a mass text message to its members urging them to sign up and vote in the contest.

The message failed to ask recipients if they consented to their data being shared by the union with Scottish Labour and it also did not ask if those signing up shared the party's values.

In the letter to Roy, Murray says the texts are clearly "against the rules of the Scottish Labour Party" and calls on the party to verify if "any sign-ups from this process are valid".

He adds: "I take absolutely no pleasure at all in writing to you in these terms but I do think we are in danger of undoing all the progress we have made in the last year because the defence of the leadership election process is, at best, clumsy and, at worst, being rigged for a particular process."

A verification panel has been set-up to ensure all those signing up as new members or party supporters in the contest are legitimate.

Separately, a sub-committee of the party's ruling executive has been established to ensure the contest's rules and procedures are being adhered to by both campaigns and external organisations.

A spokesman for the party said: "Any affiliated or registered supporter would be expected to support the aims and values of the Labour Party.

"These sign ups are eligible and within the rules of the leadership contest."

A Unite spokesman said: "Ian Murray has been told by the SLP that our members' affiliations are entirely legitimate.

"We therefore call upon him to withdraw his baseless smears now.

"Unite will be writing to the Labour party general secretary to express our concerns at these repeated and underhand efforts to bring this election and our union into disrepute."