Green Party co-deputy leader Rachel Millward has said her party would nationalise water companies in England and Wales, criticising Prime Minister Andy Burnham’s plans to bring them under public control as “vague”.

Millward told the BBC the country’s water system was an “absolutely unmitigated disaster”, arguing there had been “radical underinvestment” over decades.

She said Burnham’s plan, which involves removing limits on government ownership of shares in water companies and giving mayors more powers to scrutinise them, did not go far enough.

The Green Party of England and Wales has long called for public ownership of water companies, describing privatisation as a failed experiment.

In its 2024 manifesto, the party estimated that nationalising water companies and the five biggest retail energy firms would cost £30bn over the five years of a parliament.

Last year, the government estimated, external that nationalising the water industry would cost approximately £100bn.

Millward is expected to make a speech about the party’s latest policy on nationalising water companies – and how to pay for it – at the Green Party conference in Brighton this weekend.

It comes as the government considers the option of nationalising Thames Water, which is facing a severe financial crisis.

Ahead of the conference, Millward did a round of interviews with local BBC radio stations across England.

Under the leadership of Zack Polanski, the party’s brand of “eco-populism” has posed a challenge to Labour on the left of British politics.

Labour’s polling numbers have improved markedly in recent months, suggesting Burnham may be winning back some supporters that had drifted towards Polanski’s party.

In her interviews, Millward said the party had had a “historic successful year” by winning a by-election in Gorton and Denton, and making significant gains in May’s local elections.

She told BBC Radio Newcastle if it had not been for the rise of the Greens, then Sir Keir Starmer – who was ousted as prime minister by his own MPs – would still be in office.

“We can take the credit for that really,” she said.

Millward said Labour’s rhetoric had “shifted in our direction” under Burnham.

Going forward she said the Greens would push the Labour government to go further and continue calling for rent controls, nationalisation of water companies and wealth taxes.

But Millward faced some tough questions on local issues, some of which she wasn’t able to answer.

She was asked by BBC Radio WM about the bin strikes in Birmingham, where a minority coalition of Liberal Democrats, Greens and independents run the council.

She said she hadn’t “been given enough information to answer that question” and apologised several times.

“I’m sorry it’s not in my briefing, it should have been,” she said. “I’m happy to come on another time and chat about it.”

These BBC local radio rounds can be tricky, with presenters often asking about issues in their patches.

Millward seemed most comfortable on national topics, arguing the Greens “have the power to set the agenda for this government” and hold Burnham to account.

The Greens have five MPs and hundreds of councillors across the country.

Green Party leader Zack Polanski is currently contesting the Holborn and St Pancras by-election, which will take place next week. A full list of candidates is available on the BBC website.