JEREMY Corbyn, now the bookies’ favourite in the Labour leadership race, has received yet another timely boost with the endorsement of public sector union Unison.

As the leftwinger’s bandwagon rolls on, Ian Murray, Labour’s only MP in Scotland, will today use his first keynote speech as Shadow Scottish Secretary to give hope to his party, battered and bruised by its election drubbing, that it can revive, But he will stress that it can no longer turn to the “big beasts” of the past like Gordon Brown; rather, it must look to a “new generation to take the Scottish Labour Party forward”.

Following private polling this week, which put Mr Corbyn 20 points ahead, there appeared to be signs that his two main rivals to succeed Ed Miliband were becoming anxious at the momentum the Islington MP was building up.

Yvette Cooper, the Shadow Home Secretary, urged her party to “get serious” about the next General Election and not become simply a protest movement while Andy Burnham, the Shadow Health Secretary, said the party faced a "big choice" over "whether it is going to get straight back on a path towards...getting a Labour government back or whether we are going to be a party of protest".

After the decision to back Mr Corbyn by Unison’s 23-strong Labour Link Committee, Dave Prentis the union’s General Secretary, said the leftwinger's message had resonated with public sector workers.

"They have been penalised for too long by a government that keeps on taking more and more from them. Their choice shows a clear need for change towards a fairer society, where work is fairly rewarded and where those living and working in poverty are supported.”

Mr Prentis pointed out that while the union committee was recommending support for Mr Corbyn, Unison members were free to cast their vote for whichever candidate they thought best.

It has been estimated that some 400,000 people could vote in the leadership contest with as many as 250,000 of these being union members.

While the result is due at a special conference on September 12, ballots open in just 15 days’ time on August 14; most people, it is thought, will cast their ballots within 48 hours of receiving them.

Mr Corbyn thanked the Unison leadership for its endorsement, saying its members were bearing the brunt of the impact of the Government's austerity agenda.

"As leader of the Labour Party I would promote high-quality, modern, public services; against outsourcing, privatisation and low pay. We are building a movement for a modern, kinder Britain, and I look forward to working with Unison members to achieve that," he added.

Unison's decision, which follows a similar one by Unite, came in the wake of bookmakers William Hill, making Mr Corbyn favourite to win the contest for the first time, giving odds of 11/8, after posting him as 200/1 rank outsider when he first entered the race.

Meantime in Edinburgh this morning, at Morningside Parish Church, where the funeral of the late Labour leader John Smith took place 21 years ago, Mr Murray will seek to deliver an upbeat message to his party, recalling how it was at a low ebb in 1994 but within three years had won the first of three consecutive elections.

“Our lowest points can be the catalyst for our greatest victories,” he will declare.