LONDON (AP) — London's mayor is pledging to campaign hard to persuade British citizens to vote to remain in the European Union in a June 23 referendum.

Sadiq Khan told The Associated Press on Sunday that leaders need to to go 'door to door, street to street, city to city,' to persuade voters in a contest seen as close.

Speaking on the sidelines of Queen Elizabeth II's birthday celebrations, Khan offered an impassioned appeal to political leaders to put aside differences that have diluted the message of campaigners who want the country to remain in the EU.

"If we believe that the European Union is important for our way of life, we've got to campaign together," he said.

Khan, of the Labour Party, has campaigned with Prime Minister David Cameron, a conservative.