David Cameron’s travails over his response to Ukip’s surge has worsened after mayor of London, Boris Johnson, described the Tory manifesto pledge to cut net immigration numbers to below 100,000 as a “great deception”.

In response to its byelection success last week, Ukip is now claiming it will win the upcoming Rochester and Strood byelection, possibly forcing the prime minister from the Conservative leadership.

Nigel Farage also insisted he would require a pledge to hold an in-out referendum before August 2015 as the price for propping up a minority Tory government.

Although Farage’s talk about holding the balance of power may seem like bravado after winning only one seat, Ukip is increasingly confident that it will win in Rochester and Strood. One weekend poll put Ukip at 25% nationwide, while another put it at 15%.

A defeat in the byelection in November will raise the possibility of a leadership challenge to Cameron, or of other defections as Tory MPs calculate they are more likely to survive as Ukip candidates than Conservative ones.

Tory MPs are demanding more detail from Cameron about how he will renegotiate British membership of the EU, and a clear commitment that if he does not achieve his aims he will campaign for British withdrawal at a referendum, something he has been reluctant to provide.

Johnson, appearing on the BBC’s Andrew Marr Show, twisted the knife by claiming it had been wrong for the Conservative manifesto ever to have claimed it was possible to cut net migration to below 100,000. He said: “I think there were two big deceptions. The first was when Blair took the brakes off in 2004. Other countries kept their borders sealed and we didn’t. That was a mistake. And the second thing was saying that we could control the numbers when we couldn’t.”

He also called on Cameron to say that he will campaign against British membership of the EU if he fails in his bid to control the free movement of people within the EU.

He said: “I think it’s obviously axiomatic that if we don’t get the reform we need in 2016-17 then I think we should campaign to come out.”

He said: “UK control of its borders is now emerging as the number one thing that we need … to sort out in this renegotiation.” He urged MPs to remain calm in face of the Ukip threat and claimed in reality Ukip and the Conservatives were doppelgangers.

The Conservative rightwing MP Mark Pritchard tried to kill suggestions of a leadership challenge, saying: “Even if we lose Rochester, which we will not, now is not the time, with 200 plus days to go to the general election, for a change of leader.”

Tory HQ is assessing its chances of winning the seat before naming the date for the byelection, but will feel forced to throw everything at the campaign to reassure Cameron backbenchers.