JIM Murphy suffered a humiliating blow in the Scottish Labour leadership race yesterday, as the trade union he has been a member of for almost 20 years backed his main rival.

The GMB, which has donated almost £17,000 to Murphy's local party in East Renfrewshire since 2005, defied expectation and endorsed MSP Neil Findlay for the top job instead. The union, which has 56,000 Scottish members, also backed the Ayrshire MP Katy Clark in the party's deputy leadership race.

The GMB endorsements mean Findlay and Clark have now picked up the support of almost all of the Labour-affiliated unions in Scotland.

Unite and Unison, with more than 300,000 Scottish members between them, backed the pair last week. Although union members will cast individual votes, the recommendations from the union leadership carry significant weight, and make the contest far tighter than Murphy, the initial favourite, would like.

Union votes make up one-third of the electoral college used to pick the leader and deputy, meaning Murphy now has to win comfortably with the other two sections, the ordinary membership and the party's Scottish parliamentarians.

A spokesman for Murphy's campaign, which is run by many former Better Together staff, said: "Union leaders are entitled to make recommendations to their members but we are confident that Jim will win support from the thousands of Scottish Labour Party members and trade unionists across Scotland."

Murphy and Kezia Dugdale MSP, a candidate for deputy leader, yesterday picked up their first union endorsements, from the steel union Community. The pair are both Community members.

The shopworkers' union Usdaw, which supported Murphy's re-­election in 2005 and 2010, is likely to announce its choice today or tomorrow.

The third leadership candidate, MSP Sarah Boyack, has yet to receive any union backing.

The news that the GMB had swung behind Findlay broke as the ­Lothians MSP launched his leadership campaign in Fauldhouse Miners Club, West Lothian, providing him with his opening line and raising a huge cheer from around 150 party activists, friends and family members gathered for the event.

In a swipe at Murphy, Findlay, 45, a former bricklayer and teacher who only became an MSP in 2011, said: "I never sought to have a career in politics. I am not a career politician, I am not a machine politician."

He said that as first minister he would prioritise ending poverty and inequality, have a national living wage, ensure the NHS was fit for purpose and never privatised, and build more houses.

He said he was in favour of paying more tax on his £59,000 MSP's salary, and that the taxation system should be more "progressive".

He dismissed a report that he wanted to review universal benefits as "complete rubbish" and said he was in favour of universalism and the key issue was how to pay for those benefits - a hint he wants higher taxes on the well-off.

Although his main pitch was to the left of the party, Findlay also suggested he could win back Labour supporters who became disaffected during the independence referendum campaign, and attract new members.

As part of the event, local pensioner Jimmy Gordon said he had been a lifelong Labour voter, but voted Yes in the referendum as he couldn't see another UK government helping the working class.

"Then, like a light at the end of the tunnel, I heard that Neil Findlay was standing for the leader of the Scottish Labour Party," he said.

"He represents the kind of government I want to have looking after my interests and the interests of the ordinary people of Scotland."

In an emotional address, Elaine Holmes, 50, from Murphy's East Renfrewshire constituency, spoke of how Findlay, as Labour health spokesman, had fought for justice for her and other women who had been harmed by medical implants.

After not being interested in politics, she said she was joining Labour after seeing Findlay's "honesty and integrity" at first hand.

Murphy later issued a statement saying Labour "can, should and must" win the 2015 General Election and the 2016 Holyrood poll.

He said: "Scotland needs a confident and proud Scottish Labour Party offering a positive vision for the future, as well as exposing the cruelty of the Tories and the failures of the SNP."

John Paul McHugh, assistant general secretary of Community, said Murphy and Dugdale were "well placed to win back the trust of the Scottish people" and Murphy had "a track record of winning".