A claim by Ukip that eight more MPs are thinking of defecting to the party has been dismissed as a figment of the party's imagination by John Redwood, a former Tory cabinet minister and leading Eurosceptic.

As the former Tory leader and arch-Eurosceptic Iain Duncan Smith described Douglas Carswell as a backbench loner, Redwood said the "so-called eight" had been plucked from the dining list of the Ukip donor Stuart Wheeler who used to support the Tories.

Carswell defected from the Tories to Ukip on Thursday, promising to trigger an early byelection in his Clacton seat. However Ukip are under pressure as Roger Lord, who had been selected as the party's candidate in the seat, said he still intends to stand and accused Carswell of jumping ship to save his political skin.

Lord told the Today programme on BBC Radio 4: "It is not over yet. A few people have rung me up and told me to shut up but nobody has told me the truth behind this thus far. Douglas Carswell seems to think that Ukip is some extension of the Conservative party and is just about Europe. But we are bigger than that.

"I told him he was going to lose in May and now he has jumped ship. He has only jumped ship because the waters are around his ankles and he has got nowhere else to go."

Ukip sought to build on the momentum of Carswell's defection on Thursday by briefing that eight more MPs may be persuaded to follow his example. The briefing came as the Tory whips scrambled to contain the fallout from Carswell's defection by contacting MPs who had been identified as possible defectors.

Mark Reckless, the Tory MP for Medway, who had been seen as the most likely defector, told the Guardian on Thursday night that he would be remaining to ensure the prime minister was able to deliver his EU referendum in 2017. Reckless tweeted on Thursday morning that he had turned down a "kind invitation to lunch" from Wheeler.

Redwood dismissed the possibility of eight defectors when he told the Today programme on BBC Radio 4: "The so-called eight are a figment of Ukip's imagination. They are Mr Wheeler's dining list. Apparently now nobody should accept an invitation from Mr Wheeler – he has been the friend of quite a lot of Conservatives in the past – because then we score as someone thinking of defecting. Dream on Ukip, it's the kind of rumour you would put round if you were Ukip."

The Tories believe Ukip would not have briefed that it was lining up eight more defectors if it was confident the MPs would move into its camp. Nigel Farage, the Ukip leader, scored something of a political coup with the Carswell defection because he kept it so secret.

But the Tory whips know that many of their MPs will be watching the result of Carswell's byelection with care. If he wins well Eurosceptic MPs may think it right to jump given that they want to leave the EU anyway.

The whips appeared on Thursday night to have contained the threat of further defections in the immediate future as the former defence secretary Liam Fox, the senior backbencher Bernard Jenkin and the MP Jacob Rees-Mogg – all independent-minded Tories – distanced themselves from Carswell.

Carswell is the first sitting MP since 1982 to resign his seat to allow him to stand for another party.

"The problem is that many of those at the top of the Conservative party are simply not on our side," he said on Thursday. "They aren't serious about the change that Britain so desperately needs."

But Cameron, speaking from Scotland, warned that Carswell's defection would only benefit Labour. The prime minister said: "It is obviously deeply regrettable when things happen like this, when people behave in this way. But it is also counter-productive. If you want a referendum on Britain's future in the EU – whether we should stay or go – the only way to get that is to have a Conservative government after the next election. That is what, until very recently, Douglas Carswell himself was saying."

Carswell held his Clacton-on-Sea seat with a majority of 12,068 at the last election, but the Tories admitted they face a difficult byelection, likely to be held on 9 October, in a seat where Ukip topped the poll in the European elections in May. Matthew Goodwin, the author of a book on Ukip, predicted a Carswell victory in Clacton which he described as the "most favourable seat for Ukip in the country".

While Carswell's defection appears to be isolated for the moment, the announcement will destabilise the Tories on the highly fraught issue of Europe. There are fears that some MPs may now seek to insulate themselves from the Ukip threat by pledging to campaign for a no vote in Cameron's planned EU referendum regardless of the outcome of his negotiations.

A senior Tory MP who shared a suite of offices with Carswell at Westminster expressed surprise at his defection. Keith Simpson, who was ministerial aide to William Hague until last month, told the Guardian: "I was as amazed as anybody else. In retrospect you could perhaps see a journey. But nearly all the evidence pointed to the fact that Douglas was saying the only way we will get a referendum is to have enough Conservative MPs and indeed a Conservative majority.

"I am sure that one part of him is absolutely genuine. But I do wonder if there is an element of calculation at two levels. One is that if he does have these doubts he sees this byelection as detonator that might set off wider explosions."