Boris Johnson has mocked the “quitters, splitters and kippers” who have defected to Ukip as he re-extended a 20-year-old invitation to Nigel Farage to join the Tories.

As hardline Tory Eurosceptics accused the Ukip leader of launching a “malicious” campaign to undermine their party conference, the London mayor moved to defuse the row by calling for the “great Conservative family” to unite.

Johnson, who is going out of his way at the last Tory conference before the general election to be loyal to the Tory leadership, recalled how he and Farage tried to persuade each other to join their respective parties in a pub in the mid-1990s. Farage left the Tory party in 1992.

The London mayor told a rally organised by the ConservativeHome website: “I did meet Nigel Farage in a pub about 20 years ago, mid-1990s. As is traditional in such cases, he pushed across the caviar and the vodka that Moscow Central always used when they were trying to woo a potential defector.

“Do you know what I said to him? I said: ‘no Nigel – you join us.’ I repeat my message today because it is only the Conservatives who are able to deliver the kind of things he is talking about. 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 this country wants and deserves.”

The remarks by the mayor were double-edged for the prime minister. They were designed to be helpful by lowering the political temperature amid deep concerns among senior Tories that Farage is in the middle of a carefully orchestrated plan to disrupt the Tory conference.

But Johnson’s claim that the Tories are the only party that can deliver what Farage wants – a British exit from the EU – will delight Conservatives who want to achieve that outcome. The London mayor later spoke of renegotiating Britain’s EU membership to make it more akin to a common market.

Johnson had harsh words for Mark Reckless and Douglas Carswell, who have defected to Ukip. “I say to the quitters and the splitters and the kippers: there is only one party that is going to sort out the European issue.”

Johnson’s remarks came as Tory MPs who share much of Ukip’s thinking expressed fury with the way in which Farage, who unveiled Reckless on the last day of his party conference in Doncaster on Saturday, is trying to wreck the Conservatives’ gathering. Ukip sources had suggested further defections were on the way.

One senior Tory said: “This is completely malicious behaviour by Ukip. Their game is so obvious. But they may well have overreached themselves because Rochester and Strood [Reckless’s constituency] is friendlier territory for us than Clacton [Carswell’s seat].”

Ukip sources played down the prospect of further high level defections. The party is understood to be planning other announcements later this week, although sources stressed that these would not be at the same level as the defections of Carswell and Reckless. But Farage masterminded the Carswell and Reckless defections in great secrecy and declined to tell even members of his inner circle until hours before the public announcements.

Adam Holloway, the Tory MP for Gravesend, who had been identified as the most likely candidate for the third defection of this parliament, denied that he was planning to follow his fellow Kent MP Reckless. Holloway told the Guardian: “No sir, I have no plans to join Ukip. I am not at the conference because it is expensive and I have loads to do in Gravesend. Enjoy the warm white wine.”

The defections of Carswell and Reckless have helped to ensure that Europe, the divisive issue that has bedevilled the Tory party for a quarter of a century, is dominating the party conference before the election.

John Redwood, who stood against John Major in the 1995 Tory leadership conteston Europe, astonished centre-ground Conservatives by warning at one fringe meeting that business leaders who speak up in favour of the EU in the planned referendum would “pay a very dear economic and financial price”. He said: “The only answer for all concerned is for big business to keep out and not express a corporate view … It would be extremely foolish and we must make sure they have to pay a very dear economic and financial price were they to try that ill-judged thing.”

Alistair Burt, the former Foreign Office minister, tweeted: “Finally the J Redwood mask slips away with his threat to business which disagrees with him. I thought we were democrats? Surely misquoted?”

George Osborne highlighted the deep split that is likely to open up during the planned referendum in 2017 when he said that Conservative party backbenchers will be given a free vote.