ANAS Sarwar has called for the “spirit of the rules” of the Scottish Labour leadership contest to be respected after it was reported a mass signing by Unite of its members to the union’s political levy could swing the vote behind his rival Richard Leonard.

His comments came as Leonard was buoyed by the support of another major union, with Unison throwing its weight behind his bid to succeed Kezia Dugdale.

Leonard said he was “delighted” to have received support from the unions and added that as the rules of the contest had been agreed it was wrong to “call foul” after the decisions had been taken.

The two rivals were taking part in a hustings at the Labour Party conference following a report in the Huffington Post that Unite had signed up 2700 members to the union’s political levy in just two days last weekend – a payment that makes them eligible to vote in the leadership contest.

The Herald yesterday reported sources close to Sarwar warning Scottish Labour activists “wouldn’t take kindly” to any attempt by Unite to return the party to “a branch office”, a description used by former leader Johann Lamont when she expressed her frustrations as she quit the role in 2014.

At the hustings, Sarwar said he wanted more people to join Labour or sign up as supporters in order to vote in the contest.

But he added: “Through that process we have all got to make sure we play by the rules – that’s in terms of the written rules and also the spirit of the rules as well.

“As long as all of us do that I have no problem with more Unite members or any other trade union members joining this process, voting in this contest and hopefully then engaging with us as well as we return a UK Labour government and also a Scottish Labour government, too.”

At the hustings in Brighton, organised by the Daily Record, Leonard said: “The trade unions are going through due process of deciding who it is they are going to back and I don’t think it’s right to call foul about those rules after decisions have been taken.

“If there’s a problem with the rules you should say that there is a problem with the rules up front.”

He added Unite’s support for him followed a hustings and it was “not a decision taken by Len McCluskey”, the union chief who is close to Jeremy Corbyn, but rather by the “rank and file” of the organisation in Scotland.

At the Brighton event, Sarwar said he would not be afraid to clash with Corbyn if it was in the interests of Scotland.

“That’s not to say we are going to pick deliberate fights with the UK Labour Party of Jeremy Corbyn,” he said. “It’s about saying in a grown-up environment, with an autonomous Scottish Labour Party not being a branch office ... that we can take different decisions in Scotland that are right for Scotland’s interests.”

Announcing it’s support for Leonard, Unison Labour Link Scotland’s Gordon McKay said: “We share Richard’s vision of a society based on full employment in a sustainable economy, funding properly our public services, providing dignity for our pensioners and hope for our young.”

The food workers’ union BFAWU also declared support for Leonard, bringing to six the number of trade unions which are so far backing the Central Scotland MSP, who is a former organiser of the GMB.

Sarwar has to date yet to receive backing from any trade union.

“I am thrilled with these two latest nominations,” said Leonard. “Unison members provide many of our most vital public services and I will always stand with public service workers.

“It’ s also great to have the support of BFAWU, a union that is doing such great work and breaking new ground in bringing trade unionism to put- upon workers in the fast food industry.”

Ronnie Draper, general secretary of the food workers’ union BFAWU said: “The BFAWU is proud to nominate Richard Leonard as the next leader of the Scottish Labour Party, a trade unionist who has spent his life supporting the causes of working people. “