David Cameron uses key speech at the Conservative Spring Forum to mock Ukip

PM breaks taboo on naming Nigel Farage and his party in response to Nick Clegg's defeat in BBC's EU debate

David Cameron said that Ukip was not the party to change anything in Europe

David Cameron directly challenged Ukip's claim to deliver change in Europe yesterday as he reacted to Nigel Farage's triumph in last week's party leaders' debate on the EU.

In a highly unusual step, the Prime Minister broke the taboo of mentioning Ukip and Mr Farage by name after the Ukip leader won huge support for his clash with Nick Clegg.

An instant poll after the BBC debate gave Mr Farage 68 per cent to the Deputy Prime Minister's 27 per cent. But the resurgence for the Ukip leader – after stalling in the polls earlier this year – poses a greater threat to the Conservatives, with six weeks to go until the European elections.

Mr Cameron told activists at the Conservative Spring Forum yesterday: "Labour and the Lib Dems won't give you a say in Europe. Ukip can't change anything in Europe. It's only the Conservatives who can deliver real change in Europe, and yes – it is only us that will give the people of Britain that in-out referendum."

The Prime Minister mocked Mr Farage, saying: "Nigel Farage has admitted – I quote: 'Ukip cannot change a thing.' And frankly they barely even try. Even when they do bother to turn up to vote, they don't stand up for Britain. This time last year I sat around the table, with Merkel, Hollande, Rutte, negotiating a deal to cut the overall EU budget by €34bn. And what did Ukip do? They opposed it."

Mr Cameron's attack came as the country's oldest conservative think-tank, the Bow Group, warned that Ukip could have more members than the Tory Party within seven years.

Chairman Ben Harris-Quinney argued yesterday that the Tory party is losing ground among right-wing grassroots as it is "impossible to define our ideology" while in coalition. The group is arguing for an immediate dissolution to joint government to give the party a year ahead of the general election to "set out its stall".

Veteran right-winger Lord Tebbit added that the coalition is "beginning to smell past its sell-by date".

In his speech, Mr Cameron also hailed today's income-tax-threshold rise to £10,000 and pledged that while he was in government he would always "cut taxes where we can".

The PM said the stakes at next year's election were "truly stratospheric", and pitted a Labour Party that would "pull Britain down" against "Britain is coming back" under a Tory-led government.

Addressing the concerns of voters in marginal seats who typically say they will vote Tory because the party has more credibility on the economy, but who still regard it as nasty and uncaring, the PM said: "Let's tell them why we are dealing with the deficit. Not because we're dry fiscal calculators who relish spending cuts, but because we're parents and grandparents who don't want to pass this mess on to our children."

Mr Cameron also responded to criticism that the pro-union Better Together campaign in the Scottish referendum has been too negative. He pledged to "set out our positive vision for the future of the UK".

Meanwhile, Alex Salmond, Scotland's First Minister, is expected to mock the UK's record on education and life expectancy as he presses the case for independence tomorrow in New York during Scotland Week, which celebrates the country's culture in events across North America.