David Cameron promised a Thatcher-style “single-minded” approach to Europe yesterday as Gordon Brown prepared to tell him of the dangers of standing on the sidelines.

The Conservative leader widened his pledge of a referendum on the EU reform treaty, saying that the Tories would hold one if they gained power before it was ratified in all countries, even if it has already been ratified by the British Parliament.

But he stopped short of meeting the wishes of party Eurosceptics who want him to promise a referendum even after the treaty has been ratified by the rest of the EU, a position that ministers claim would lead to a Tory government being forced to pull out.

Mr Cameron, interviewed on The Andrew Marr Show on BBC One, conceded that it would be difficult to have a referendum if all 27 countries had, by the time the Tories gained power, ratified it. However, he said that he would not be happy to let “matters rest there” and suggested that he would try to pull Britain out of the EU’s social and employment policies.

Asked whether he could offer a “cast-iron” guarantee that he would offer a referendum when in government, he said: “As soon as we have an election, the sooner we can have a referendum.”

He added: “I think Margaret Thatcher showed in recovering the British rebate that if you have a very clear, very straightforward approach in Europe and say look, we want to be in the European Union . . . but we’re not happy with the status quo and there are some things we want to change, if you’re single-minded about that, then there’s no reason why you cannot achieve your objectives.”

He said that a referendum could take place with “absolutely no problem at all” while it was still being discussed in Europe. But he admitted that it would be more difficult if there were no general election until 2010 and if all member states had ratified it without holding a referendum.

“We will not be content to rest at that point because we think too much power will have been passed from Westminster to Brussels,” he said.

Speaking to business leaders in London today Mr Brown will give warning against casting any doubt about Britain’s membership of the EU. The Prime Minister will say that at a time of global economic uncertainty, “we should not be throwing into question our future membership of the EU - risking trade, business and jobs”.

He will continue: “I strongly believe that, rather than retreating to the sidelines, we must remain fully engaged in Europe so we can push forward the reforms that are essential for Europe’s, and Britain’s, economic future.

“The EU is key to the success of business in the UK: Europe accounts for nearly 60 per cent of our trade; 700,000 British companies have trading ties to Europe and 3.5 million British jobs depend upon Europe. And even in the face of rapid globalisation, our trade with Europe continues to rise, meaning Europe is as important to the future of Britain as ever.”

Jim Murphy, the Europe Minister, said Mr Cameron’s policy was in complete confusion. “Attempting to reopen negotiations on a treaty ratified by every member state would put the Tories into the extraordinary position of repudiating an international treaty, reopening and renegotiating Britain’s terms of EU membership.

“David Cameron admitted that he is already committed to reopening negotiations on Britain’s position in Europe over employment and social policy. The consequence of such a policy would be years of economic instability that would be gravely damaging for British business, British jobs and British prosperity.”

Times Recommends

• Should Knox’s trial even have reached the courtroom?
• All that glitters is not sold in Wright’s world
• Believe it or not, Big Brother is your friend