The United Kingdom "should be prepared" to leave the European Union Boris Johnson has said, as David Cameron condemned Tories "pessimists" who doubted his plan to renegotiate the terms of membership would succeed.
On Thursday former cabinet minister Michael Portillo became the latest senior Conservative to call for Britain to leave the EU.
Writing in The Times today, Portillo said: "The default position of the political class is defeatism: the belief that Britain could not survive outside the union; and the political class assumes that the public shares its defeatism."
And he savaged Cameron's plan to hold an 'in or out' referendum on membership should he win the 2015 general election. "To put it mildly, you could not imagine Margaret Thatcher approaching the issue in such an insincere and political way," he said.
Portillo added: "Nigel Lawson says that he would vote in a referendum for Britain to leave the European Union. So would I."
On Tuesday former Tory chancellor Lord Lawson also told The Times that he had decided it would be best if the UK left the EU. His comments energised anti-EU Tory backbenchers, many of whom want to see the prime minister introduce referendum legislation before the general election.
Last night Boris Johnson, widely seen as wanting to succeed Cameron as Tory leader also flirted with eurosceptic members of his party. He told an audience in North London last night Britain "should be prepared to leave if we cannot get what we want" from a renegotiation.
In a speech on Thursday morning, the prime minister hit back at Portillo, Lord Lawson and other Tories who have said it will be impossible to renegotiate a looser relationship with Brussels and said he was "optimistic" about his chances of success.
"You simply have to change and reform this institution. It’s in Britain’s interests to be uniquely well-connected. Then Britain will have a choice whether to stay in. It’s the optimistic approach. It’s absolutely achievable and deliverable," he said.
David Cameron is under pressure from eurosceptic Tory MPs
The prime minister defended his approach of seeking fundamental reform and holding a referendum on membership after 2015 as "logical, sensible, practical".
"I am faced as I do so, if you like, by two groups of pessimists," he said. "There are some pro-European pessimists who say, 'you have to, in Europe, simply sign-up to every single thing that anyone in the EU suggests. You sign every treaty, you sign everything - there is no alternative'.
"I think they are completely wrong. The second group of pessimists say there is no prospect of reforming the EU, you simply have to leave. I think they are wrong too."
It was also claimed this week that Margret Thatcher decided Britain should leave the EU following the ratification of the Maastricht Treaty in 1992. However according to Charles Moore, the former prime minister's biographer, she was persuaded to keep her view secret.
Portillo's conversion to the anti-EU cause was welcomed by leading eurosceptic Tory MP Douglas Carswell who tweeted: "And another one joins the Better Off Out position. Things fall apart, the center cannot hold ... should have had an In Out Bill in QS today eh?"
The internal-Tory wrangling over how to deal with Ukip overshadowed yesterday's Queen's Speech, as Ed Miliband warned Cameron not to try and "out-Farage Farage" to please his backbench MPs.
Some Tories have concluded that the only way to prevent the haemorrhaging of more votes to Nigel Farage's party is to form an alliance. Peterborough MP Stewart Jackson said "an electoral pact with Ukip is an imperative to prevent a Labour or Lab-Lib Government".