A powerful trade union headed by the controversial Len McCluskey is likely to back left-winger Richard Leonard to be the next Scottish Labour leader, the Sunday Post understands.

Members’ votes will decide which candidate receives Unite’s official backing, with meetings and hustings to take place in the coming weeks.

But sources said the influence of General Secretary Mr McCluskey, a key ally of UK Labour boss Jeremy Corbyn, was likely to prove decisive in who ends up receiving the union’s support.

Mr Leonard has been a consistent and public supporter of Mr Corbyn’s leadership, defying the former leadership team at Holyrood to back him during a failed coup last year.

One union insider said: “Given the national position, particularly that of the general secretary in support of Jeremy Corbyn, if you looked at the cards it would be likely they will fall in favour of Richard Leonard.

“There is due democratic process that has to take place but if you were a gambling man I would suggest Richard Leonard is the most likely to be supported.”

Such an outcome would be a blow to the other leadership contender, Anas Sarwar, a Unite member.

He is seen as being in the moderate wing of the party, with Mr Leonard more to the left and closer politically to Mr Corbyn. Mr Leonard has already been backed by union chiefs at the Transport Salaried Staffs’ Association.

The union’s executive committee unanimously gave its support to the former GMB union organiser, saying his “radicalism” could help the party win back power.

Mr Leonard and former MP and previous deputy leader Mr Sarwar are the only two candidates to succeed Kezia Dugdale, who dramatically quit her post a fortnight ago after two years in charge.

Both candidates launched their official campaigns this weekend, with Mr Leonard addressing activists in Glasgow yesterday.

He ruled out any electoral pacts with the SNP, an idea floated by Labour MEP David Martin, saying: “There will be no ground ceded to nationalism at the expense of progressive socialism under my leadership.

“No coalition, pacts, or deals with the SNP. And no second independence referendum.”

Mr Leonard also claimed that “wholesale, real, and radical change” was needed to tackle Scotland’s “economic, social and environmental problems” .

Mr Sarwar said Labour was “parking our tanks on Nicola Sturgeon’s lawn” when he launched his bid on Friday night.

Speaking in the SNP leader’s constituency, he told supporters that the contest was about electing the next First Minister.

Mr Sarwar told supporters they could choose “to focus on the divisions of the past, or to build a fairer future for all”.

SNP MSP James Dornan branded the contest the “least inspiring choice in the history of devolution”.

The result will be announced in November.