David Cameron said his most potent weapon in taking on a resurgent UK Independence Party in the eastern region next month is his promise of an in/out referendum.

Launching his European election campaign yesterday the Prime Minister said it was not in Britain’s national interest to walk away from Europe, but to fight for a better relationship.

But he would not be drawn on how confident he was that the Conservatives would be able to win three seats again, saying he was not a pollster or a pundit.

At the last European vote the Conservatives won three seats and UKIP won two.

David Campbell Bannerman, who stood for UKIP but defected to the Conservatives in May 2011, will be fighting for the third seat this time.

The Tory campaign launch yesterday comes after a torrid week in which culture secretary Maria Miller was forced to resign amid a row over her expenses.

Questioned on whether he would apologise to grassroots activists in Suffolk and Essex, who will have to answer questions on the doorstep amid polls which said Mrs Miller had to go, he said: “The lessons to learn as I said in the House of Commons this week, is that expenses anger is still raw, and we properly respect that.

“We also have to get across that all post-2010 expenses are not going to be dealt with by MPs. They are being dealt with by an independent organisation. They are being dealt with now. When people hear that they feel hugely reassured that the MPs are not sitting in judgement over each other’s expenses claims.”

He said: “You have to make difficult decisions in this post. I don’t think the right thing to do is to fire someone at the first sign of trouble. The right thing to do is give people the opportunity to clear their name, she had that opportunity and then get on with their jobs and that I think is the right thing to do.”

Mr Cameron said that the Conservatives Party was a very positive alternative to the other parties fighting for European seats, saying: “I think the most potent weapon is based on three words – Britain’s national interest. It is not walking away from Europe.

“Britain’s national interest is to fight for a better relationship and give people a choice. The more people hear that, the more they realise the Clegg/ Farage debate was a totally false choice – one person who won’t change anything, and one person who can’t change anything.”