THE Scottish Labour leadership contest is in crisis after one of the party’s most senior figures said the election process is being “rigged” to help left-wing candidate Richard Leonard.

In an incendiary letter to party headquarters, Edinburgh South MP Ian Murray called for Labour to re-consider the validity of the members of the Unite trade union who had signed up to vote in the contest.

He also said revelations about interim leader Alex Rowley getting caught backing Leonard raised questions over “impartiality” and called on him to be removed as chair of a panel set up to vet new recruits.

Murray wrote: “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.”

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A Unite spokesman said: "Ian Murray is wrong. Unite has adhered to the SLP's rules to the letter, and indeed the general secretary and the party's legal and governance unit have all confirmed that our affiliated members are valid.

"We urge Mr Murray to take Brian Roy's advice on this and in doing so withdraw his wrongful accusations about Unite."

A senior Labour source described the Murray letter as a “massive diversionary smokescreen” created to take attention away from the member recruitment strategy pursued by supporters of the other leadership candidate, Anas Sarwar.

The source added: "These are the actions of a crumbling establishment."

Meanwhile, in another development, Leonard was last night nominated by the Edinburgh Southern Constituency Labour party, which has Murray as a member.

Leonard and Sarwar are facing off in the race to succeed Kezia Dugdale, but the battle has become one of the dirtiest leadership contests in years.

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A key flashpoint relates to the party’s decision to allow new members and supporters to join and get a vote in the election.

Three routes were available until Monday of this week: joining as a full member; signing up as a registered supporter; or becoming a supporter through an affiliated trade union.

It was claimed that up to 2,700 members of Unite, Labour’s biggest donor and a Leonard backer, may have signed up through the third method.

However, there was also reported unhappiness about the text message Unite had sent its members that led to the potential sign-ups.

Murray, who is backing Sarwar and sits on the party’s governing Scottish Executive committee (SEC), has written to general secretary Brian Roy with concerns.

He wrote: “There is no doubt whatsoever that the way in which Unite has signed up affiliate members to vote in the leadership election is against the rules of the Scottish Labour Party and the process.”

Murray said there are “serious questions” to ask about the Unite sign-up process: “The text clearly failed to ask the recipients if they supported the aims and values of the Labour Party, nor did it ask them to consent for their data to be shared with the party.

“In order for this process to be fair and robust, the party must reconsider whether any sign-ups from this process are valid.”

When the contest began, Rowley said he would not endorse a candidate, but he was covertly recorded expressing support for Leonard. The interim leader has also been linked to plots against Dugdale, which he strenuously denies.

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Murray wrote: “The additional issue is the sanctity of the newly formed verification panel. It was a good idea for the SEC to initiate this process but it cannot be allowed to continue to function with the Interim Leader of the Scottish Labour party overseeing it given the revelations last week that question both his impartiality and his role in undermining Kezia Dugdale.”

The Murray row is separate from a controversy reported earlier this week by the Herald, over claims that new members had signed up with the same email addresses and mobile phone numbers.

A spokesperson for Scottish Labour 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 spokesman for the Sarwar campaign said: “If concerns have been raised about an element of the process, the party has a responsibility to investigate and ensure the same strict rules are followed by all involved.”

A spokesperson for the Leonard campaign said: "Trade unionists have had a direct role in Labour leadership elections for decades but now have to meet higher hurdles than ever before to participate: we welcome their involvement as a means of connecting Scottish Labour to workplaces and communities across the country."