David Cameron travels to Brussels today for a showdown over his EU reform demands.
Here we look at what could happen over the coming weeks and months:
Talks on the reform package will reach a potentially decisive stage when the Prime Minister meets leaders from across the 28-strong bloc at Thursday's European Council summit to thrash out a deal based on proposals put forward earlier this month.
The wrangling could run into the weekend but, if the package is approved, it will pave the way for a swift referendum.
Mr Cameron is believed to be keen to hold a vote on June 23, as a quick poll would avoid giving Eurosceptics time to make headway with voters and pre-empt a repeat of the migrant crisis in the Mediterranean and eastern Europe over the summer months.
Mr Cameron will call a Cabinet meeting on Friday if he secures an early deal on his renegotiation of Britain's EU membership.
If the summit runs into Friday evening or Saturday, the PM will review whether to call Cabinet ministers in to Number 10 for a rare weekend meeting.
The Cabinet meeting will effectively fire the starting gun on the referendum race, as Eurosceptic ministers will then be allowed to campaign for a Leave vote.
Technical work will also begin, with secondary legislation setting the date and rules for the vote likely to be laid in Parliament rapidly.
The Electoral Commission will rule on which groups are designated the main Remain and Leave groups for the campaign, which must last at least 10 weeks, allowing them access to higher spending limits.
But as the battle to win over voters begins in earnest, Mr Cameron will also be fighting for support in his own backyard. The Prime Minister started out his leadership determined to avoid a repeat of the deep divisions over Europe which left the party on its knees in the 1990s but, even with the dispensation allowing Cabinet ministers to speak their minds during the campaign, he will face a major challenge to stop an all-out war.
Commons Leader Chris Grayling and Work and Pensions Secretary Iain Duncan Smith are expected to be prominent Brexit campaigners. Northern Ireland Secretary Theresa Villiers is also reported to be considering voting to leave. Home Secretary Theresa May, who had been tipped as a possible leader of the Out campaign, has signalled she could be prepared to support the reform proposals.
Both camps are eagerly waiting to see which way London Mayor Boris Johnson will jump.
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.
If the UK votes to sever ties with Brussels, it will trigger a process expected to last two years, with attempts to negotiate key trade deals with either the entire EU or bilateral agreements with each nation.
A referendum defeat would mean Mr Cameron coming under intense pressure to quit as Prime Minister.
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.