Voters have been given their first sight of the European Union referendum ballot paper as the Government published detailed rules - keeping the door open to an in/out vote in June.
The proposed form is shown in draft regulations laid before Parliament on the conduct of the national poll, one part of the legislation which must be approved at least 10 weeks before the UK is asked to decide whether to remain in the EU.
The timing suggests the Government has not ruled out an early vote - though David Cameron has sounded increasingly cautious notes about his prospects of securing a renegotiation deal in time.
Former prime minister Tony Blair warned that a vote to leave the EU would be followed by a second independence referendum north of the border which would see Scotland quit the UK.
In an interview on French radio station Europe 1, Mr Blair said: "There is a little-noticed dimension which is that, in my opinion, if the UK votes to leave Europe, Scotland will vote to leave the UK. It is extremely serious for Great Britain."
Speaking in French, Mr Blair said he expected Mr Cameron to secure the package of reforms to British membership which he is seeking, and to lead the Remain camp in the referendum campaign.
"There are big strategic and economic reasons to remain in Europe," said the former PM.
"I am worried because there will be a vote and it is always possible that people will vote for leaving Europe. But I hope not and I believe not."
February's European Council summit is the deadline for holding a referendum before the summer, but the Prime Minister has acknowledged there remains work to do to win over fellow leaders, notably on moves to curb EU migration to the UK.
Reports today suggested June 23 was Downing Street's preferred date for a snap referendum - though that would coincide with a scheduled EU summit, a potential headache for the PM.
A Downing Street spokesman said: "I'm not going to speculate on specific dates. The key focus is on the negotiations and getting the best deal for the British people, and that is what we are doing."
The spokesman declined to comment on reports that Britain and Germany are at loggerheads over welfare for migrants.
Chancellor Angela Merkel is understood to have floated a tightening in the EU's legal definition of a worker, after resistance from a number of states to Mr Cameron's proposal of a four-year ban on in-work benefits for new migrants.
Mr Cameron spoke to his German counterpart by telephone last night, with No 10 saying the pair agreed there was "more work to do ahead of the February European Council to find the right solutions".
The Times reported that chancellor Merkel is suggesting a qualification threshold of 20 hours a week to be counted as a worker, which would equate to around £7,000 a year on the UK's minimum wage, while Britain was reported to be pushing for a figure closer to £13,000.
Downing Street said: "We are not giving a running commentary on the negotiations. Our proposal on the table is still the four years, and that is what we are discussing."
By law, the referendum must be held by the end of 2017, with autumn 2016 still seen as favourite if a deal is not agreed in February.
Separate orders setting down the conduct of the vote, the process for designating the principal campaigner on each side and for the timings have to be approved at least 10 weeks before polling day - though the Electoral Commission recommends it should be six months.
A June referendum would therefore fall outside the watchdog's "best practice" advice - which is not legally binding.
The proposed ballot paper is headed "Referendum On The United Kingdom's Membership Of The European Union" and reminds voters to put a cross in only one box.
Beneath is the question: "Should the United Kingdom remain a member of the European Union or leave the European Union?" followed by the two options: "Remain a member of the European Union", or "Leave the European Union".
The question was changed from a simple yes/no formula on the recommendation of the Electoral Commission.
In another step towards the referendum, campaigners have been told they can register with the Commission - and must start recording all donations and loans above £7,500 - from February 1.
The watchdog invited groups planning to spend more than £10,000 promoting one side or the other to submit applications after the Government announced that the European Union Referendum Act would come into force from the start of the month.
Spending before the start of the formal campaign does not count towards statutory limits but donations above the threshold will be published in pre-poll reports by the Commission.
It means campaigns must also check donations and loans are from eligible sources.
Bob Posner, director of party and electoral finance and legal counsel at the Commission, said: "Campaigners are central to the referendum and it is important that their sources of funding and spending are transparent to voters.
"Having these provisions in place is part of enabling this."
The Commission said it would publish an "updated assessment of progress and readiness for the referendum" once the required orders were laid that would "identify any specific issues associated with the proposed regulations at that point".