Keir Starmer has been urged not to backtrack on his plans to spend £28bn on green projects by Labour’s Greater Manchester mayor, Andy Burnham.

He told the Labour leader to “stick to your guns” after Starmer said he may scale down the investment given the financial picture he would inherit if he became prime minister.

Burnham said the money would be “investment in the future”, as Starmer faces pressure over the planned annual green payments.

The Conservatives have been increasing their attacks on Labour over the proposal after the UK entered the election year, but supporters have been concerned about watering down the pledge. Burnham told LBC’s Tonight With Andrew Marr: “I would absolutely say to the party: stick to your guns, you’re on the right path.”

The mayor said the investment could reindustrialise the north and make the nation a “clean energy superpower”, adding: “That is about building future prosperity. This is not spending; this is investment in the future, investment in new industry, investment in better homes for our residents, better transport.”

Burnham said the investment could be raised incrementally rather than spent upfront but warned that “the more we delay” the more other governments will win the green race.

Jeremy Hunt said: “You have to pay debt interest on the £28bn whether you call it ‘spending’ or ‘investment’”. The chancellor added: “Money is money … and on this scale can only lead to tax rises.”

Labour promised in 2021 to invest £28bn a year until 2030 in green projects if it came to power. But last year the shadow chancellor, Rachel Reeves, said it would instead be a target to work towards in the second half of a first parliament, should Labour win an election.

Last week Starmer described the policy as a “confident ambition” rather than a commitment. He said that ensuring Labour sticks to its “fiscal rules”, which include making sure debt is falling, was more important than hitting the green spending target.

“That means that if the money is from borrowing, which it will be, borrowing to invest, that if the fiscal rules don’t allow it, then we will borrow less,” he said.