David Cameron's chances of securing a rapid EU renegotiation deal face a key test as Donald Tusk arrives in the UK to finalise a package to put to other leaders.
The European Council president is due at Downing Street for a working dinner to thrash out the details of proposed reforms which he is expected to publish within days, ahead of a summit in February.
Top of the agenda will be changes to strengthen a proposed "emergency brake" on European Union citizens claiming welfare in the UK - which Mr Cameron will say must come into force immediately after an in/out referendum.
The measure has been put forward by Brussels as an alternative to Mr Cameron's plan to impose a unilateral four-year curb which other member states ruled out as discriminatory and in breach of the freedom of movement principle.
The PM cancelled a trip to Scandinavia yesterday to fly to Brussels to discuss the idea with European Commission president Jean-Claude Juncker and European Parliament president Martin Schulz.
He said the mechanism was "not good enough" in its current form and a senior Government source said he will seek assurances that present levels of migration will be deemed sufficient to trigger it and that it could remain in place "long enough to resolve the underlying problem", perhaps up to seven years.
Mr Cameron will insist that it should only be regarded as a "stop gap" while a more permanent solution was worked on.
A deal at the February 18-19 summit is seen as vital if Mr Cameron wants to hold an early referendum on EU membership, because an agreement at a later date would make it hard to schedule a vote before the school summer holidays.
Mr Cameron however insists he will not do a deal "at any price" and is prepared to hold off with the plebiscite - which must be held by the end of 2017 - if he considers the deal on the table to be inadequate.
Eurosceptics have dismissed the proposals on the table as "pretty thin gruel" that would do little or nothing to stem the flow of would-be workers arriving in the UK, notably from eastern European states.
It is thought that the proposed benefit ban would be available to all EU states, and be activated when migration levels were deemed high enough to put public services or welfare systems under severe strain.
It is one of four areas where he is seeking changes to the UK's relationship with the EU, alongside the lifting of member states' commitment to "ever-closer union", measures to protect non-euro states and improve EU competitiveness and greater powers for national parliaments.
He is expected to press for the current proposals in all four to be strengthened further as well as for action to tackle " back door" immigration into the UK and other "abuse" of free movement rules.
It came as shadow home secretary Andy Burnham complained that the campaign for Britain to remain in the UK lacked "passion and fight" and was suffering from a "fetish" with using business voices to make the case.
He said immigration - not economics - "will be the decisive issue of this campaign" and that the UK would stand more success of securing agreement from the rest of the EU if it played a fuller role in dealing with the refugee crisis.
"The only saving grace is that the Leave campaign this week looks in even more disarray," he wrote in the Observer.
"So we have a chance to get our act together. But there can be no delay. Brexit means break-up. If we want to stop it, we must start campaigning now with honesty, purpose - and passion."
The "in" camp has been boosted by a £250,000 injection from Conservative Party donor Andrew Cook, who has also agreed to serve as Treasurer of the new Tory "In" campaign, Conservatives for Reform in Europe, the Sunday Telegraph said.
He told the newspaper he was "deeply concerned" that so-called Brexit would damage his business, which makes components for armoured vehicles, trains and generators.
"If we come out of the EU, it is foolish to assume that we will still enjoy free trade with its members," he said.
"There is already hostility to the freedom with which British manufacturers can sell their products against local competition. Don't expect them to stand idly by and allow this to continue if we leave."
The recruitment of party donors has renewed suspicions among some anti-EU Tories that the party machine - which has made a commitment not to campaign for either side - is encouraging donors to back "remain".