“My intuition is that treaty change is close to mission impossible today because it’s not only about rationality, about good argument,” said Tusk.

He was referring to the required unanimity among the governments and 28 national parliaments to modify the EU treaty.

“We have to help David Cameron because he is obviously pro-European. I am sceptical when it comes to changing the treaty,” he said.

Cameron has promised to hold a nation-wide referendum in 2017 should he be re-elected as PM in elections in May. Amid a heated domestic political debate about immigration, Cameron has promised to clamp down on the rights of EU migrants to social welfare.

The tough talk is seen, in part, as an attempt to woo voters away from Nigel Farage's Ukip party, which has made immigration a key issue.

There has been much speculation in the media about whether Cameron's Conservatives and Farage's Ukip would form a coalition together.

Farage, for his part, said he is willing to form a post-election pact with the Conservative government if Cameron agrees to the hold the EU membership referendum before the end of the year.

"I want a full and fair referendum to be held in 2015 to allow Britons to vote on being in or out of the European Union," said Farage in the Telegraph newspaper.

The four million EU nationals living in Britain should be banned from taking part in the poll, he said.

Farage has also said he will step down from the Ukip leadership if he fails to secure a seat as MP at the House of Commons.

“It is frankly just not credible for me to continue to lead the party without a Westminster seat,” he said.

Farage was elected with a four-year term as party leader last November.

The latest developments comes amid a new report by the Open Europe think tank, which says Britain would still face high annual costs even if it is no longer in the EU.

The Open Europe findings suggest the UK would still pay some €44 billion a year – around 94 percent of the current costs – if it had a similar relationship with the EU as non-member state Norway.

“While the ‘Norway option’ does mean greater independence in certain areas – chiefly the repatriation of agricultural policy, regional policy, trade policy and justice and home affairs – overall, it would make little sense to leave one club only to join another with many of the same costly rules,” says the report.