Burnham recently suggested £500k political donation cap that would be ‘reduced over time’

Exclusive: Campaigner says PM wrote in email he believed £500k cap should be reduced over time to guard against undue influences

Andy Burnham recently supported an even lower donations cap than previously reported, according to correspondence seen by the Guardian, as the row over next week’s elections bill escalates.

The prime minister wrote to a campaigner in May outlining his support for a £500,000 individual donations cap, which he said would “guard against the perception of any one party being unduly influenced or swayed by one person or organisation”.

In a follow-up email seen by the Guardian, Burnham went on to say he believed the cap should gradually be reduced – a proposal now being put forward by the Labour MP Stella Creasy .

Campaigners are accusing Burnham of a U-turn after the Guardian reported the prime minister had decided not to back the attempts by Creasy and others to introduce a donations cap in next week’s elections bill.

In his first message to Shaun Bowler, who runs the website wakeupgb.co.uk , Burnham wrote: “I do think there should be a cap on political donations. This would guard against the perception of any one party being unduly influenced or swayed by one person or organisation.

“As to the level, I think that would have to be subject to wider review of political funding but my gut feeling would suggest somewhere in the region of £500k.”

In a second message, the then would-be MP for Makerfield said: “[I] would definitely want to see that reduced over time. I am in complete agreement that there needs to be wholesale culture change in Westminster.”

Bowler has now written to the prime minister accusing him of abandoning his previous commitments. He wrote: “It appears you are going to break your promise to me – and the nation …. Weeks after being elected, you have made your first major U-turn.”

He added: “This is your chance to actually deliver the start of that cleanup. Show us that politics can be clean and trustworthy. Don’t surrender to the mess.”

Other campaigners, including the broadcaster Carol Vorderman, are also urging the prime minister not to back down in the face of opposition from trade unions, who have warned that a donations cap could undermine their ability to contribute to the Labour party.

Senior figures at groups including Transparency International, Spotlight on Corruption and Greenpeace have also written to Burnham. In an open letter, they call for a “low and hard cap on all political donations”, adding: “We know you agree that politics should not be for sale. Please take action now to ensure it will not be under your leadership.”

The issue will come to a head next week when MPs debate the next stage of the elections bill. The bill will lower the voting age to 16 and cap donations by Britons based abroad, but not on domestic donations.

Creasy had put forward an amendment to cap domestic donations at £100,000, while her colleague put forward an amendment to cap them at £1m.

Creasy has now changed her amendment to set the cap at £500,000, reducing in the second year to £100,000 unless ministers intervene to set it at a different level. The proposal, which has cross-party backing, mirrors what Burnham told Bowler he would support in May.

The prime minister’s aides say that while he is not backing a donations cap next week, he is not ruling one out in the future. A Labour source said: “This bill was designed to deliver our manifesto commitments on votes at 16 and to strengthen political donation rules to guard against foreign interference and that’s what the government is prioritising. But we’re not ruling out taking further steps to protect our democracy in the future.”

Officials also said the government would not support an attempt by the Labour MP Anneliese Dodds to reduce the amount political parties can spend during an election campaign after the limit was raised by the previous Conservative government.