Media playback is unsupported on your device Media caption Boris Johnson: "This is a great time to be a Conservative"

Boris Johnson has urged the "great Conservative family" to unite to defeat Labour at the next general election and to guarantee a referendum on Europe.

Speaking in the wake of a second Tory defection to UKIP, the mayor of London said only the Conservatives could "sort out" the UK's relationship with the EU.

He told a Conservative conference rally that the "ideal solution" would be for the UK to stay in a reformed EU.

But he suggested the UK would also prosper in a "trade zone" outside it.

Former Tory MP Mark Reckless' defection to UKIP on the eve of the Tory conference has increased pressure on David Cameron.

The move, which came less than a month after Douglas Carswell's departure to UKIP, has caused "unconstrained fury" in the Tory leadership, BBC political editor Nick Robinson said.

'Changed everything'

Speaking at a fringe meeting at the Conservative conference in Birmingham, Mr Johnson said he had never been approached about defecting to another party although he joked that "he had once met Nigel Farage in a pub about 20 years ago".

And he sought to rally support behind David Cameron and his pledge of a referendum on the UK's membership of the EU in 2017 if the Tories win the next election.

"It is only if the great conservative family unites and we stop Ed Miliband seizing back control of this country that we will be able to deliver the referendum that this country wants and deserves," Johnson told activists.

Image copyright PA Image caption Mr Johnson said Labour was "virtually unelectable"

Image copyright Getty Images Image caption Douglas Carswell (left) stood down as MP for Clacton to stand in the resulting by-election for UKIP

Image copyright PA Image caption Mark Reckless made the same move on Saturday, to the delight of UKIP and its leader Nigel Farage (right)

"I say to the quitters, the splitters and the 'Kippers, there is only one party that can sort out the European issue."

Mr Johnson, who will officially address the conference on Tuesday, urged the party to unite to win a working majority next year, suggesting that under Ed Miliband's leadership Labour were "virtually unelectable".

"When you look at the vast leads that this party enjoys on the key questions of the economy and prime ministerial qualities, I think they (voters) will come over in droves," he said.

And referring to one of Margaret Thatcher's landslide election victories in the 1980s, he added: "If we get this right, this election campaign could be more like 1983 than 1997."

Earlier the Eurosceptic Conservative MEP Daniel Hannan told the BBC's Daily Politics Mr Reckless and Mr Carswell had "made a mistake" in leaving the party.

"I think they have both acted on principle - nobody does this kind of thing lightly," he said.

Mr Hannan said he would not be following in their footsteps and joining UKIP, because David Cameron's pledge of a referendum on the UK's membership of the EU had "changed everything".

'Respectful'

Speaking on BBC Radio 4's Today programme, former Environment Secretary Owen Paterson said the Conservatives had to be "absolutely clear" to people who could switch to UKIP.

He added: "We have got to be respectful of them and we have got to make a very clear, intelligent case that the only national organisation that can resolve the long-term problems of the UK is the Conservative Party."

But former Chancellor Ken Clarke said the Conservatives should make the argument that UKIP is "rather a nasty organisation and its basic case is folly".

Speaking at a fringe debate, he criticised the "neurotic" debate about Europe which had, he said, "rather dominated our party for the last 20 years".