Labour’s energy policy has been become mired in confusion after Keir Starmer blindsided shadow business secretary Ed Miliband by ruling out nationalising the big six energy companies.

It is understood Miliband was frustrated by Starmer’s comments, as he had been keen to keep open the option of nationalising parts of the energy sector to aid the economy’s transition to net zero.

Appearing on BBC One’s Andrew Marr Show on Sunday, hours before Labour delegates passed a radical motion calling for a “socialist green new deal”, Starmer was asked directly whether he would consider nationalising the main energy providers to tackle the energy crisis. He replied: “No.”

Instead, speaking shortly before Miliband was due to deliver his conference speech, he said Labour would advocate nationalisation only in certain circumstances.

“When it comes to common ownership, I’m pragmatic about it,” he said. “Let me spell it out. What that means is that where common ownership is value for money for the taxpayer and delivers a better service, then I’m in favour of common ownership.”

By contrast, Miliband suggested on Newsnight earlier in the week that the party was about to renew its commitment to common ownership of energy and other public resources.

“We haven’t changed that commitment,” Miliband said. “If we’re going to make this green transition, then public ownership is the right way to go.”

The shadow business secretary has not yet set out which elements of the system Labour would take into public ownership – but it is understood he was not ready to rule out nationalising energy providers, preferring to set out details closer to the next general election.

The “big six” controlled about 70% of the energy supply market in 2019.

A senior Labour source, asked whether Miliband or Starmer were setting out Labour’s stance, said the party’s policy was “what Keir says it is”.

The Labour leader’s comments came as delegates at the party’s conference in Brighton overwhelmingly backed a motion calling for the party to adopt a series of radical climate policies.

As well as nationalisation of the energy sector, these included the creation of a “national nature service,” “a government programme creating millions of well-paid, unionised green jobs with publicly owned entities,” and “mass investment in green technologies and renewables”.

Gaya Sriskanthan, Momentum co-chair, said: “This is a turning point. The grassroots have had enough of timid centrism and have overwhelmingly endorsed transformative socialist policy that meets the crises of the 21st century head on.”

Starmer’s comments on the big six appeared to contradict his own stance when he was seeking Labour members’ votes during last year’s leadership contest.

One of his 10 key pledges was that “public services should be in public hands, not making profits for shareholders. Support common ownership of rail, mail, energy and water.”

Sriskanthan said: “Starmer has spent long enough running away from his 10 pledges. It’s time to support transformative policy. Labour members back bold solutions – the leadership needs to follow suit.”

In his conference speech on Sunday, against a backdrop of queues outside petrol stations and empty shelves in supermarkets, Miliband reminded Labour members of David Cameron’s warning: “Britain faces a simple and inescapable choice – stability and strong Government with me, or chaos with Ed Miliband.”

“I’ll tell you one thing: I’d have done a damn sight better than this miserable shower,” he said.

Miliband said it was Labour’s responsibility “to be the party of green and red together. To be the party of climate and economic justice together.”

Chris Saltmarsh, co-chair and co-founder of party pressure group Labour for a Green New Deal, said: “It’s fantastic to hear Labour talking about building a vision around a green new deal, but it must back up words with policies.

“As new polling shows, a majority of the public are in favour of renationalising our energy companies. This is the perfect opportunity to build on that support and deliver a unifying, ambitious programme for climate justice.”

Starmer will make his own speech on Wednesday, the first in person since taking over the leadership last year, after kicking off the gathering with an internecine wrangle about how his successor will be chosen.

Starmer announced a new policy on Sunday of scrapping the charitable status of England’s private schools, and spending the money on giving children more soft skills to make them more work-ready.

Starmer said: “This is a political choice to take that money and switch it to our state schools so that children and young people in our state schools have the best chance they can have to come out of schools ready for life, ready for work.”