Cameron welcomed Labour Party mayor Khan to the rally. The two men have agreed to put their differences aside, during they campaign for Britain to stay in the EU.

Khan said: "There are many things on which the prime minister and I will disagree. But what's really important is when it's in Londoners' interests for the mayor and the government to work closely together, we will work closely together."

Khan told the rally that more than 500,000 jobs in the British capital depend on the country being a member of the EU.

Cameron joins hands with London mayor to back Remain campaign

As well as unveiling the battle bus, Cameron and Khan revealed a "five point guarantee card" listing what they say Britons will get if they vote to Remain.

Their card promises full access to the EU's single market, protection of workers' rights, "a safer Britain", Britain maintaining its "special status" within the EU, the option to say no to EU aim of "ever-closer union".

The Vote Leave camp shuffled their alternative pack of cards showing the consequence of a remain vote. This included the mush disputed claim that Britain is sending 350 million pounds (512 million U.S. dollars) a week to the EU, and a warning that the free movement of people across the EU will continue.

Former Prime Minister Tony Blair has also joined the campaign, saying if Britain quits the EU, it will cause an enormous economic problem.