Boris Johnson is expected to back a Brexit at 10pm tonight

Boris Johnson is expected to back a so-called Brexit - British exit from Europe - in his Daily Telegraph column published online at 10 p.m. GMT (5 p.m. ET) on Sunday.

The Daily Telegraph is reporting that the Mayor of London and Member of Parliament will reveal his stance on the upcoming referendum on European Union membership in his weekly column for the paper.

Johnson is understood to be a euroskeptic and several political journalists are tipping him to back the "leave" campaign, including The Sunday Times political editor Tim Shipman and ITV's Robert Peston.

A City financier who advises Boris Johnson on pensions has also attacked David Cameron's EU reforms as "no more than a whingers' charter", according to an email seen by Sky News. It suggest the mood around Johnson is anti-EU.

Several high-profile Tory MPs have already come out in favour of Britain leaving the EU, including Justice Secretary Michael Gove and Secretary of Work and Pensions Iain Duncan Smith.

Duncan Smith went as far as to claim in an interview with the BBC that staying in the EU "exposes the UK to terror risks."

It presents a problem for Prime Minister David Cameron, who supports Britain staying in the EU. Cameron is understood to have directly appealed to Johnson in a bid to try and win him over.

The battle lines within his party have been drawn after Cameron announced the expected referendum on Europe will be held on June 23.

Johnson's expected backing of a Brexit would be 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 that 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.