London Mayor Boris Johnson speaks to the media in front of his home in London, Britain February 21, 2016. Britain will hold a referendum on European Union membership on June 23. [Photo/Agencies]

LONDON - London Mayor Boris Johnson, a heavyweight conservative, announced Sunday that he will back the call for Britain to leave the European Union in a national referendum on the country's EU membership.

The announcement was regarded by the British media as a blow to Prime Minister David Cameron's efforts to keep Britain in the EU as Johnson was an eloquent speaker and popular among voters.

Cameron announced Saturday that Britain would hold a referendum on June 23 to decide whether to stay or leave the European Union and recommended British people to choose remaining inside the EU, following a deal with leaders of other EU countries which gives UK a "special status" in the community.

Johnson, who Cameron had sought strongly for support, said that he came to his decision after a "great deal of heartache" and he wanted "a better deal for the people of this country".

He said that the prime minister had done "fantastically well" to reach agreement with other EU leaders, but no one "could realistically claim that this is fundamental reform of the EU".

"I will be advocating Vote Leave ... because I want a better deal for the people of this country to save them money and to take back control," Johnson said.

Cameron is scheduled to speak in the British parliament Monday to kick off the campaigns before the referendum. Opinion polls have shown that the British public is almost evenly divided while many ruling Tory politicians, including six cabinet members, will side with the so-called "Brexit" camp.