An extra European summit could be held in late February to finalise a deal on David Cameron’s EU reforms, which would allow the UK prime minister to hold a referendum on Britain’s EU membership in June.



Amid increasing optimism that a deal is within reach, sources familiar with the negotiations suggested that the European council president, Donald Tusk, may exercise his power to call a further EU summit if progress is insufficient at the next meeting due in mid-February.



“Donald Tusk has the power to call a special European council at any time,” one source familiar with the negotiations told the Guardian. “It is not necessary to wait a full month between the February and the March councils for EU leaders to meet.”

Earlier in the day, Jean-Claude Juncker, the European commission president, said the negotiations were entering their final weeks when he said he was quite sure that “a permanent solution” would be reached next month. Speaking at a press conference, Juncker said: “I am quite sure that we will have a deal. Not a compromise, [but] a solution, a permanent solution in February.”

Holding an extra summit would be seen as a gesture to Cameron amid fears in No 10 that if a deal was not reached until the following European council in March the referendum would have to wait until September, by which time a full-scale summer migration crisis could be at its height.

Officials say approximately 16 weeks must pass between the conclusion of a deal between Britain and the other EU members and the referendum. Reaching a deal in March would delay the referendum until September. July has been ruled out because the Scottish school holidays are under way then.

Tusk has said the prime minister’s plans to introduce measures to discourage migrants from travelling to the UK are the most contentious in the negotiations. But Juncker echoed the warning from UK government ministers against seeing the other issues as easier to resolve.

Agreement on handing the UK an opt-out from the EU’s historic commitment to create an “ever-closer union”, allowing national parliaments to club together to block EU legislation and to provide guarantees for non-eurozone countries would also be difficult to agree, Juncker said.

Senior British figures say they have detected a distinct mood change in Brussels and national capitals after Cameron’s 45-minute speech to the European council in December. This persuaded some wavering EU leaders that he was genuinely seeking to keep the UK in the EU on better terms.

There is intense focus on the welfare “basket” in the negotiations, which includes Cameron’s most controversial plan to ban EU migrants from claiming in-work benefits for four years.

Discussions are taking place within Whitehall and between Downing Street and other EU capitals to allow the prime minister to say he has achieved his main goal on EU migration – a reduction in “the current very high level of population flows from within the EU into the UK”.

There were suggestions during the week that the prime minister had secured agreement on an even better deal than his four-year ban by winning approval for an “emergency brake”.

This would allow the UK and other EU members to limit levels of migration, subject to agreement from the European commission, if they are able to prove that public services are facing unbearable strains in their countries. Officials say that a version of this idea is being kicked around but is not formally on the table.

Also under examination is a proposal to ban access to in-work benefits for one year, an idea which has been highlighted by Angela Merkel.

British officials are cautious about this idea on the grounds that it would be quite a climbdown. There are also concerns that EU migrants could mount legal challenges.

There is growing support in Germany for a one-year waiting period. Merkel hinted as much at a press conference with the Romanian prime minister, Dacian Cioloș, on 7 January where she cited an idea expressed both by the German labour minister, Andrea Nahles, and by Olaf Scholz, mayor of Hamburg.

However, discussions are not yet finalised. Officials from two other eurozone governments have separately indicated that even a one-year curb could be problematic for them.

In any case, the EU is set to introduce a waiting period before migrants can access in-work benefits as part of a package of labour market reforms the commission is hoping to publish in March.

Downing Street is adamant that a four-year restriction on tax credits remains on the table, although the prime minister said at a press conference in Budapest earlier this month that he is open to listening to alternative solutions.