EU President Donald Tusk arrived in London on Sunday for talks over dinner with Prime Minister David Cameron on Britain's demand for EU reforms, with Tusk expected to release his proposed response to the demands in the next few days.
Tusk wrote on Twitter that he will "present solutions" on all four areas in which Cameron is seeking reforms, while adding that there will be "no compromise on fundamental freedoms."
Cameron said he had made progress in his negotiations after meeting European Commission President Jean-Claude Juncker in Brussels on Friday, but he warned that EU proposals to address his demands were still "not good enough."
There has been fierce media speculation around the proposals, notably on Cameron's demand to restrict in-work benefits for EU migrants, including talk of an "emergency brake" that could be applied if Britain can show that its welfare system is overwhelmed.
A senior British government source said Cameron planned to tell Tusk that the emergency brake must apply immediately after Britain votes in an in-out referendum on the terms of its continued EU membership.
Cameron would also call for "greater ambition" from EU leaders on all four areas where Britain seeks reform: competitiveness, sovereignty, social security and economic governance.
How a so-called emergency brake would work is expected to be the main topic at Sunday's dinner in Downing Street, after EU officials conceded last week that EU nationals should have to pay into Britain's welfare system before receiving benefits, the source said.
"This [concession] has opened the way towards a solution but the PM will make clear that a deal will only be possible if a brake would apply to current levels of EU migration to Britain, could be triggered immediately after the referendum and would apply long enough to resolve the underlying problem," the source said.
Freedom of movement is one of the most cherished achievements of the EU and is enshrined in its treaties. Many are wary of changing the treaties in order to meet Britain's demands, as this can be a long and complicated process.
"I don't expect Tusk to offer future treaty change on free movement," an EU source said on condition of anonymity, adding that Tusk was accompanied to London by his chief negotiators, including his top legal advisor.
Cameron has promised to renegotiate the terms of Britain's EU membership before holding an in-out referendum by the end of 2017.
Speculation has mounted that he could announce a referendum in June if he secures a satisfactory deal at an EU leaders' summit on Febnruary 18-19, but the British government source said Cameron would not rush into a deal.
"The prime minister intends to leave Tusk in no doubt that he will not do a deal at any price, making clear he is not in a hurry and that it's far more important to secure significant reforms," the source said.