London's new mayor, a Labour Party member, is joining forces with Prime Minister David Cameron in the campaign to prevent a British exit from the European Union.

Sadiq Khan is making joint campaign appearances Monday with Cameron, the Conservative Party leader who also favors remaining part of the 28-nation bloc.

Campaigning is intensifying ahead of the June 23 in-out referendum with Cameron's Conservatives badly split over the issue of Europe. He is opposed by several Cabinet members who favor leaving the EU.

Khan will join Cameron on a "Britain Stronger in Europe" bus trip. He said the issue is more important than political differences between the two men.

Labour Party leader Jeremy Corbyn has refused to make joint appearances with Cameron, even though they agree on the EU issue.