London's Mayor Boris Johnson speaks on the third day of the Conservative Party Conference in Manchester northern Britain, October 6 , 2015.
Speaking live outside his home in North London on Sunday, the Mayor of London and Member of Parliament said that he believes Brits should vote to leave the EU in the upcoming referendum on membership on June 23.
Johnson is one of several high-profile Tory Party members to have backed a so-called "Brexit" - a British exit from the EU - over the weekend. Justice Secretary Michael Gove and five other cabinet secretaries have called for Britain to leave the union.
It presents a problem for Prime Minister David Cameron, who himself supports Britain staying in the EU. Cameron is understood to have directly appealed to Johnson in a bid to try and win him over.
Johnson's backing of a Brexit is a significant win for the "Vote Leave" campaign, the official campaign group calling for a Brexit. A poll released by the Evening Standard last week found one in three voters regard Johnson's opinion as "important" when it comes to deciding which way to vote.
Zac Goldsmith, the Tory Mayoral candidate hoping to replace Boris in City Hall, on Sunday said he would vote for Britain to leave the EU, the BBC reports.