Nigel Farage has urged Boris Johnson to campaign alongside Ukip ahead of the planned referendum on the UK's membership of the EU. The Ukip leader, marking his Westminster return with a speech in London, said he was not sure where the Mayor of London stood on the issue and would need "damascene conversion" to call for Britain to leave the 28-nation bloc.

"I would say, 'come on Boris, see the light, come on in – the water's lovely'," Farage quipped. Johnson, a Tory minister who is tipped as a future leader of the party, would be a big coup for the Brexit campaign.

The former magazine editor is reportedly in favour of a two-referendum plan, according to The Sunday Times. The offer could see the public voting against the prime minister's renegotiation package, but having the opportunity to vote to stay within the EU in a bid to get more concessions with Brussels.

Farage, who failed to win the South Thanet seat at the general election, promised that Ukip would "lead on the ground" in the Brexit campaign and urged eurosceptics to campaign alongside the purple party.

'Yes really means Yes'

"This is the referendum that no one thought was going to happen. It's the referendum that the prime minister hoped he would never have to deliver and it's the referendum that Ukip have spent 20 years fighting for," he said.

"My goodness me, aren't the Yes side busy? Aren't they active? They're campaigning at full throttle and never a day goes by without Sir Richard Branson appearing on the TV."

Farage added: "But be in no doubt that if we vote Yes in this referendum, it really means Yes. It will be a vote for deeper political union, it will be a vote for more EU legislation, it will be a vote for an extension of EU foreign policy and if the Yes men get their way, it will be a vote that brings Britain closer to joining the euro."

The Ukip leader also joked about Labour's leadership contest, which has seen a surprise surge by left-winger Jeremy Corbyn and interventions from party grandees like Tony Blair. "It is terrific. This afternoon I'm not sure whether to watch that or the Ashes," Farage said.

Tory and Labour MPs have set up their own Eurosceptic campaigns – Conservatives for Britain and Labour for Britain – but the groups have promised to wait until Cameron's renegotiation is over before siding with No or Yes.

Meanwhile, George Osborne has travelled to France to kick off a European tour in a bid to woo his continental counterparts into agreeing to concessions for the UK.