A POTENTIALLY damaging left-right split opened up in the Scottish Labour leadership contest yesterday, as Scotland's biggest union snubbed Blairite MP Jim Murphy and backed the left-wing MSP who is his main rival.

Bosses at the Labour political wing within Unison, which has 160,000 members, agreed to recommend MSP Neil Findlay in the contest to replace Johann Lamont as the party's leader.

Transport unions Aslef and TSSA also endorsed Findlay to their members.

Other unions are expected to recommend members back Findlay, Labour's health spokesman at Holyrood, instead of the ex-Scottish secretary.

Unite's political committee is likely to make its recommendation on ­Tuesday. Unite Scottish secretary Pat Rafferty dropped a strong hint his union would back Findlay when he attacked the speech Murphy made at his campaign launch yesterday.

Urging Murphy to spell out policies for tackling poverty, low pay and public services, Rafferty said: "On the basis of this speech, it is extremely difficult … to find much hope that Jim Murphy is offering … genuine, positive change in Scottish Labour."

A senior Labour insider hit back at Rafferty, saying: "Whoever wins this campaign will be leading the fight in Scotland to get the Tories out of Number 10 next May and Pat Rafferty should probably reflect on that."

It will be up to individual union members to vote, but endorsements from the union bosses carry considerable weight, and indicate Murphy faces a challenge from the left of Labour.

Findlay has set himself against Trident and backed greater devolution, including more say over income tax. Murphy is against unilateral nuclear disarmament and cagey on tax powers.Unions and other affiliated societies make up one-third of the electoral college to decide the next leader and deputy. Party members and Scottish MPs, MSPs and MEPs get the other votes.

Although Unison is the biggest union north of the Border in membership, it is second in terms of leadership votes, as only 40,000 of its members pay a political levy to Labour. With 150,000 Scotts members, most paying the levy, Unite is the main union in the contest.

Unison's decision came after Findlay and the other leadership candidate, MSP Sarah Boyack, addressed the union at its Glasgow HQ yesterday as Murphy launched his campaign in Edinburgh.

Insiders said Boyack's pitch was about strategy for electoral recovery, while Findlay characterised his as "policy, policy, policy".

Gordon McKay, chairman of Unison Scotland's Labour Link committee, said Findlay had been hugely impressive.

"Neil understands that politics as usual isn't good enough and we believe he offers a fresh approach with a real experience and understanding of the concerns of working people."

Aslef general secretary Mick Whelan said Findlay had the "character, vision, talent and ability" to deliver change.

Murphy is expected to pick up some union backing, with shopworkers' union Usdaw, with 45,000 Scottish members, and the iron and steel union Community, with 3000 members, strongly tipped to back him. Murphy will also try to woo his own union, the GMB, with 56,000 Scots members.