New battle lines have been drawn in the race for the London Mayoral election as candidates pin their colours to the Vote Leave or Stronger In European Union campaigns.

On Saturday, Prime Minister David Cameron announced that an In Out referendum on Britain's membership of the European Union would be held on June 23.

February 20: Date for EU referendum has been set - how will you be voting?

Today: Zac Goldsmith reveals £10m earnings in the past five years as tax returns published

Within a day, the current London Mayor Boris Johnson declared he would be joining the Vote Leave campaign.

Zac Goldsmith, the Conservatives' candidate to replace Mr Johnson, will also campaign to leave the EU.

In an opinion piece for City AM, he said: "The most important characteristic of any democracy is that it should be possible for voters to evict those who act in their name. The EU fails on that test. Real power in Brussels rests in institutions that are accountable to no one.

"I recognise that opinion in London is at best divided on this issue, and as a mayoral candidate, it would be easier for me to quietly U-turn. But I didn’t get involved in politics to test every idea with pollsters, flip flop on the big issues and then carry on regardless once elected. That characterises my Labour opponent’s approach to politics, not mine."

Mr Goldsmith said he was "on the side of the challenger" claiming that made small to medium size businesses find it hard to hire because of EU regulation.

Mr Goldsmith's Labour competitor Sadiq Khan said that the Conservative's position was a threat to London.

He said: "By placing dogma ahead of Londoners’ interests, he has chosen to jeopardise our place as a global city, put our safety at greater risk and sacrifice our ability to build a better future for all Londoners. Zac Goldsmith is advocating a position that is clearly damaging to the people he is seeking to represent.

"London needs a Mayor who will campaign for Britain to stay in Europe. I am running to be Mayor because London is at a crossroads - and the future of our city is at stake. We need a Mayor who will campaign for Britain to stay in Europe - and a Mayor who will make our city better for all Londoners."

UKIP London Mayoral candidate Peter Whittle said that Mr Goldsmith's position still meant London needs a "strong UKIP voice".

He said: "It has been a great pleasure to see both Boris and particularly Zac Goldsmith as my competitor in the Mayoral race come round to the position that we in UKIP have held for decades. We believe that Britain, and London in particular would be better off out of the European Union.

"We believe that the structures of the EU are ill designed for the innovation and entrepreneurship that has been the mainstay of London for centuries. It is slow and heavy, demanding simplistic and bloated rules that stifle the quick thought and quick action that is the hallmark of an exciting successful and prosperous city."