Jim Murphy was last night dragged into claims of dirty tricks in an increasingly fierce Scottish Labour leadership contest, after facts which could have helped his main opponent were not supplied to voters.

The super-union Unite wrote to Labour's UK general secretary, Iain McNicol, demanding to know why trade union nominations for the leader and deputy had been excluded from publicity material sent to eligible voters.

Unions overwhelmingly backed Murphy's left-wing rival, Neil Findlay MSP, for the leadership, and left-wing MP Katy Clark as deputy. But the material sent by the party to voters contained only nominations by MPs, MEPs and MSPs - the bulk of whom backed Murphy for leader and MSP Kezia Dugdale for deputy.

Also omitted were nominations from constituency Labour parties, slightly more in favour of Murphy and Dugdale.

A senior union source said: "It does not feel particularly clean as a process. Jim Murphy is an establishment figure within the Labour Party. He will have very good relationships with the party at a high level."

In a letter to McNicol, Unite's Scottish secretary Pat Rafferty said: "You can imagine my surprise when I looked at the candidate election booklet to find that my union's nomination, along with all the other trades unions, had been omitted from the publicity material. This strikes me as a gross error of judgment. To intentionally or otherwise exclude the nominations of the trades unions feels disrespectful to all trades union members in Scotland."

Rafferty added last night: "We are urgently seeking clarification from the Labour Party to find out why this has happened and if needs be will join other unions and affiliated organisations in making a formal complaint.

"Unless the party deals with this situation urgently they will stand accused of trying to skew this election for favoured candidates."

Findlay said he was also seeking clarification. He said: "Over the last 24 hours I have been made aware of a deep frustration from local Labour Party branches and affiliated organisations that their nominations for the leadership campaign have been excluded from the candidates booklet … It is insulting given the effort and expense members, constituency Labour parties, trades unions and socialist societies went to organise special meetings and consultation events, only to be ignored."

A spokesman for Murphy declined to comment. Meanwhile, a spokesperson for Scottish Labour said candidates must have at least 10 nominations from MPs, MSPs and MEPs which are included in the candidates booklet.

Supporting nominations are a separate process.

The spokesperson added: "The deadline for supporting nominations was three days before ballots were despatched so there was no way to include them in the candidates' booklet."