London's new mayor has joined forces with Prime Minister David Cameron in the campaign to prevent a British exit from the European Union.

The Labour Party's Sadiq Khan and the conservative Cameron made an unusual campaign appearance together Monday just weeks after Khan won a bitterly fought election campaign to become London's first Muslim mayor.

Both made passionate pleas for voters to reject a British exit, or "Brexit," in the June 23 in-out referendum.

Khan's appearance represented a break with Labour Party leader Jeremy Corbyn, who has refused to make joint appearances with Cameron even though he agrees with the Conservative Party leader on the need to stay in the EU.

Khan said he will work with the Conservative-led government when it is in Londoners' interests. He said the economic reasons to remain in the 28-nation EU bloc are clear and that there is an important "patriotic case" as well.

"This vote is about our values, it is about our character, it is about how we see our city and our country in the future," he said. "The reason why London is the greatest city in the world — and it is — we have never taken an isolationist approach. We are open-minded, we are outward-looking. We embrace other cultures and learn from other cultures and ideas as well."

Cameron, whose own Conservative Party is deeply split by the issue, said an "extraordinary coalition" had formed in favor of staying in the EU.

Although he had strongly opposed Khan during the campaign, Cameron said Khan's victory shows that Britain is on the right track in terms of embracing diversity.