DAVID Cameron has cast serious doubt about whether an EU referendum will be held this summer after he adopted a clear change of tone, saying he was in “no hurry” to secure a reform package at the Brussels summit next month.

An agreement at the European Council on February 18-19 is widely seen as crucial if the Prime Minister is to stage, as he said he wanted, the in/out referendum in June.

But speaking at the World Economic Forum in Davos, Switzerland, Mr Cameron stressed that he was ready to be "patient" in order to get the right result.

Securing a deal next month was "achievable(and) doable", he insisted but noted: "We are certainly not there yet."

Speaking to an audience of international political and corporate leaders, the PM claimed Britain's demands for change on the four issues of migration, sovereignty, competitiveness and protection for non-euro states were "not outrageous asks" but offered "a huge prize".

He explained success in the renegotiation could deliver the UK "the best of both worlds" by allowing it to be within the single market while retaining control of its borders and remaining outside the single currency and the ever-closer union sought by some other member states.

"That is a prize worth fighting for, it's a prize worth negotiating for, if necessary it's a prize that we will have to be patient in order to achieve but it's a prize I'm determined to deliver in this, my second term as Prime Minister," declared Mr Cameron.

He told the forum that there was goodwill among EU partners to get a lasting agreement.

“If there’s a good deal on the table, I will take it. But if there isn’t the right deal, I’m not in a hurry. I can hold my referendum any time up until the end of 2017.”

This is a clear change of tone since the beginning of the year when the PM was upbeat about securing a deal next month, paving the way for a summer poll.

Last week, Mr Cameron confidently indicated that an historic deal was close, saying he wanted a deal in February and a referendum in June.

Earlier this week, David Mundell, the Scottish Secretary, claimed there was "no argument" against holding the EU poll in June.

After a string of PM bilaterals with EU counterparts, No 10 said everyone had agreed about the “importance of maintaining momentum towards getting a deal done; noting that there was a good understanding of the main issues and, with hard work, solutions could be found in all four areas" of proposed reform.

However, significantly perhaps, there was no mention of a deal being struck next month in Brussels.

At the Swiss ski resort, Mr Cameron issued a plea for businesses and charities not to delay making the case for continued EU membership until after the negotiations were completed.

“It's important that with this, which is such a massively important generational question for Britain and for Europe, the sooner you can start to look at your own businesses and come up with the examples and the ideas about the benefits and the problems that there are with Europe, the more you are able to help to explain and set the context for this vitally important question for Britain and for Europe," he said.

Elsewhere, Alan Johnson, the former Labour Home Secretary who chairs the Labour In For Britain campaign, said: "It's crucial British businesses large and small back the fight to keep British jobs by staying in Europe.”

But Liz Bilney, Leave.EU chief executive, said: "The fact the Prime Minister is begging for international corporations to stick their noses into our democratic process only shows how little he and the elite care for ordinary British citizens. This referendum is about our needs, not theirs.”

She added that now the pro-EU camp was being backed by the likes of Wall Street bankers Goldman Sachs and JP Morgan, the EU debate was “gearing up to be the Establishment versus the people".