David Cameron's friendship with Boris Johnson damaged by EU debate

David Cameron has said his friendship with Boris Johnson has been affected by the EU referendum debate Credit: PA

David Cameron's friendship with Boris Johnson has been damaged by the two taking opposing sides in the EU referendum, the prime minister has revealed.

In an interview with Glamour magazine, Cameron - who has been friends with Johnson since their days at Eton and Oxford University - said he had hoped for a "reasonable, civilised argument" over the issue.

The prime minister is campaigning to keep Britain in the EU, while the mayor of London backs Brexit.

He said it was "frustrating" that Johnson - and another close ally, justice secretary Michael Gove - had chosen to campaign on the opposite side in the battle over the UK's European future.

Boris Johnson is backing Brexit Credit: PA

"I think they have made the wrong decision, but they're politicians and they have to decide," he said.

In the interview - in the June issue of the magazine due to be published on Thursday - Cameron also reiterated he would not quit as prime minister if the country backed Brexit and said he did not expect the debate to end if voters supported staying in.