Andrea Egan, the general secretary of Unison, threw the UK’s largest trade union behind Energy Secretary Ed Miliband for chancellor of the exchequer on Thursday, intensifying the battle over the Treasury as Andy Burnham moves toward becoming prime minister. Egan said Miliband was the only candidate among those reported to be in the running who could deliver the policies working people need.

“Andy Burnham has a historic opportunity to rebuild our country in the interests of workers and communities,” Egan said. “We need a chancellor who will rewire the economy and properly invest to improve the lives of the majority. Of those reported to be in the running, only Ed Miliband could enact the kinds of policies trade unions and our members urgently need.”

The endorsement from Unison, which represents 1.3 million members primarily in the public sector, opens a split with two other large unions – GMB and Unite – that oppose Miliband. A senior union official told the Financial Times on Thursday: “There are ongoing discussions to try to stop Ed Miliband. There is a GMB-Unite axis on this.”

Unions with strong representation in the North Sea oil industry have been exasperated by Miliband’s refusal to back down on his promise not to issue new licences in the North Sea, and also fear he will not approve the Jackdaw and Rosebank megafields. Progressive economists earlier this week pushed back on Unite leader Sharon Graham’s claim that Miliband would destroy jobs, arguing the climate transition is a major driver of industrial job creation.

The tussle between Miliband and his most likely rival, former health secretary Wes Streeting, comes as Burnham prepares to give his first major policy speech since becoming MP for Makerfield. He will speak in Manchester on Monday about devolution and the economy. Burnham has chosen James Purnell as his chief of staff, a move that angered some on the left due to Purnell’s Blairite roots.

The next chancellor will inherit a loaded in-tray: high debt, low growth, welfare reform, defence spending demands, and decisions on the de-privatisation of water and energy utilities. Streeting is seen as more business-friendly and more likely to reassure international investors; Miliband is seen as more ideologically sympathetic to Burnham’s agenda. Some investors believe Miliband would be anti-business, pointing to his past rhetoric about “producers” versus “predators.”

Burnham is in the process of assembling his ministerial team, having been elected as an MP a week ago. Following Starmer’s resignation announcement on Monday and Streeting’s endorsement of Burnham, it is now overwhelmingly likely that Burnham will enter No 10 as soon as next month. Labour’s national executive committee confirmed Thursday that if only one candidate secures enough support to be on the ballot – 81 Labour MPs – a new leader will be named on July 17.

Current Chancellor Rachel Reeves has indicated she would like to stay in position, telling the BBC on Wednesday: “I’m not going to pre-empt the decisions that the new prime minister will make.” She later told the British Chambers of Commerce that she hoped whoever succeeds her would stick to her policies, arguing they were beginning to “bear fruit.” Burnham’s allies, however, said he would not keep Reeves in place.

Other potential candidates for chancellor include Home Secretary Shabana Mahmood, Foreign Secretary Yvette Cooper, Work and Pensions Secretary Pat McFadden, former Defence Secretary John Healey, and Chief Whip Jonathan Reynolds. Smaller unions, including the TSSA, are expected to back Miliband in the coming days, and the National Education Union also endorsed him Thursday.