David Cameron has vowed to "battle for Britain through the night" as he meets EU leaders - hoping to push his case for reforms.
But the Prime Minister has already had a warning from the European Council President - Donald Tusk - that his demands on migrant benefits are 'unacceptable'.
Mr Cameron wants restrictions on the ability of other EU citizens to claim welfare within four years of moving.
The Taoiseach Enda Kenny has referred to the idea as an attempt to change "one of the key principles of the European Union" while adding that there is "very strong support" for Britain to remain in the European Union.
The British Prime Minister David Cameron is pressing for his demands for EU reform at a leaders summit in Brussels.
But he acknowledged no deal would be reached over night but said it was important to driver for "real momentum".
The European Commission chief Jean-Claude Juncker has warned EU reform has to be fair for all countries, not just Britain.
Mr Juncker said there were other options to be considered and not just the ones Mr Cameron has tabled in his list of four demands.
The key sticking point has been a four-year ban on UK in-work benefits for EU migrants, which European leaders are opposed to because it threatens the key EU principle of freedom of movement.
Mr Juncker said: "We want a fair deal with Britain and this fair deal has to be a fair deal with other countries".
Mr Juncker added: "We'll enter the concrete and vital phase of negotiations with our British colleagues. The Commission is ready to look for other options than the single one proposed by the British prime minister and I'm quite convinced that we will find a solution to that highly complicated question".
Speaking in Brussels, Mr Kenny said those discussions - along with migration issues - will be "quite contentious".
Four UK demands
Over dinner tonight, Mr Cameron will have his first significant opportunity to gauge support for the concessions he is seeking for Britain.
Bilateral meetings have revealed backing for some elements of the reform package, but negotiations are expected to be tough.
He is essentially making four demands in his renegotiation of the UK's deal with the 28-nation bloc.
He wants protection for EU countries who have not joined the Euro, reforms to promote competitiveness, and guarantees over UK sovereignty and the benefit curbs.
It is seen in key European capitals as contrary to the fundamental principles of EU freedom of movement and non-discrimination, and therefore requiring a change to core EU treaties.
The main EU powers have ruled out treaty change now for political reasons.
Mr Cameron has invited his European counterparts to offer alternative policy reforms that can help the UK limit large flows of migration from other EU countries since the advent of the Eurozone crisis.
The European Council President Donald Tusk has asked for a political debate "with no taboos" ahead of a formal deal at the next summit in February.
In the intervening period, EU and UK officials are expected to thrash out the technicalities of a compromise over migration.
One option floated involves an emergency brake on the payment of benefits to EU migrants if flows become excessive.
