David Cameron's bid to secure backing for an EU reform deal has "reached crunch time", a UK official has said, as the Prime Minister prepares for a key summit with politicians in Brussels on Thursday.
The stakes are high for Mr Cameron, who requires a good deal to secure the backing of senior ministers - with ITV News Political Editor Robert Peston reporting that two, Boris Johnson and Michael Gove, are already considering backing an exit from the union.
So what's next for the PM in his EU negotiations?
What is happening today?
Cameron is travelling to Brussels for a showdown over his EU reform demands.
There he will meet leaders from across the 28 EU member states to try and thrash out a deal based on proposals he put forward earlier this month.
What does Cameron want?
Cameron's four main objectives are:
Economic governance - Safeguards for the UK so they will not have to contribute to eurozone bailouts and reassurances countries outside the eurozone are not financially disadvantaged
Immigration - Tighter restrictions placed on EU migrants' access to benefits, including a rule they have to be living in the UK for four years before they are eligible to claim
Sovereignty - Giving Britain the choice to opt out and block EU legislation and exempt us from forming an "ever closer union"
Competitiveness - Calls to cut red tape on business and repeal unnecessary laws which are a burden for the UK
Is he confident?
The Prime Minister has said he is "optimistic" about securing a deal and that there is "good basis for agreement".
But EU officials have made clear that key details of his reform package remain to be nailed down.
European Council president Donald Tusk has also warned there is "no guarantee" a deal will be reached.
What will happen next?
Wrangling could run on into the weekend but, if the package is approved, it will pave the way for a swift referendum.
Cameron will call a Cabinet meeting on Friday if he secures an early deal effectively firing the starting gun on the referendum race.
The Electoral Commission will then rule which groups are designated the main Remain in the EU and Leave groups for the campaign, which must last at least 10 weeks.
If no agreement is reached at the February summit, there will be a second chance on March 17.
Although an emergency meeting could be called before then to preserve some chance of a referendum before the summer.
Who will represent the In and Out campaigns?
Commons Leader Chris Grayling and Work and Pensions Secretary Iain Duncan Smith are expected to be prominent Brexit campaigners.
And ITV News understands the Mayor of London Boris Johnson and Justice Minister Michael Gove are on the verge of declaring they will campaign to leave the EU, in opposition to the Prime Minister.
Home Secretary Theresa May, who had been tipped as a possible leader of the Out campaign, has now signalled she could be prepared to support the reform proposals.
When will a referendum take place?
Cameron is said to be keen to hold a vote on June 23.
What will happen if we vote to leave the EU?
If the UK votes to sever ties with Brussels, it will trigger a process expected to last two years.
This will include attempts to negotiate key trade deals with either the entire EU or bilateral agreements with each nation.
A referendum defeat would also mean Cameron would come under intense pressure to quit as Prime Minister.
And if we stay?
A vote to remain in the EU will not signal the end of battles with Brussels, with key parts of the deal requiring legislation in the European Parliament.
The parliament's president, Martin Schulz, has promised to be "constructive" but could give no guarantees that the deal would be signed off without challenge.
And Mr Cameron has been warned that there is a "distinct possibility" that some groups within the parliament might seek to wreak havoc on the deal.