Burnham accused of U-turn after initially backing political donations cap to fall below £500k

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".