Agreement on the UK's renegotiation of its relationship with the EU may be delayed beyond next month's Brussels summit by the pressure of other urgent problems facing the 28-nation bloc, Foreign Secretary Philip Hammond has suggested.
Mr Hammond said it would by "inappropriate and unwise" for the UK to insist on sealing a deal at the European Council summit on February 18-19, which will be held amid the continuing migration crisis.
But he acknowledged that a June date for the in/out referendum promised by David Cameron will become "much more difficult" if no agreement is reached in February, and "impossible" if there is no deal at the following summit in March.
Speaking to the House of Lords EU Committee, Mr Hammond confirmed that, although the referendum could be held at any time before the end of next year, the UK Government wants the renegotiation settled "well before" the French and German elections in 2017.
His evidence came as voters were given their first sight of the European Union referendum ballot paper in documents tabled by ministers in Parliament, which set out detailed rules for the vote and leave the door open to a June referendum.
The draft regulations must be approved at least 10 weeks before the UK is asked to decide whether to remain in the EU.
The timing of their production 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, and Downing Street declined to comment on suggestions he has pencilled in June 23 as a favoured date.
Asked whether he believed a deal in February was possible, Mr Hammond told the Lords committee: "It is much more important to get the right deal than to get a quick deal, and we have to recognise the fact that the EU has got a couple of other issues on its plate.
"Also, the British negotiation is number one on our European agenda, but I'm afraid it isn't number one on the European agenda, I think it's fair to say, of any of our partners, except perhaps for the Irish.
"The reality of the situation is we have to recognise that there are a lot of other very important issues chewing up the bandwidth and chewing up time at the Council ...
"I think it would be inappropriate of us and it would also be unwise of us to try to push our agenda ahead of other things which the other EU member states will see as being pressingly urgent to discuss at the February Council.
"But we are still confident that there will be a substantive discussion on this issue. We are hopeful that between now and then we will be able to get to a position where we can get that agreement at the February Council."
Mr Hammond left no doubt that the Government is aware of the potential difficulties of delaying a vote to 2017, when France holds presidential elections in April and May and Germany its parliamentary elections between August and October.
The Foreign Secretary said: "We are focused on the fact that there are major French and German general elections in 2017, and if this was not resolved during the course of 2016, we would have to think very carefully about how the realities of those election campaigns interacted with this debate, so we would certainly hope that it would be resolved well before then."
Asked whether failure to secure a deal in February would make a vote before the summer impossible, Mr Hammond said: "If we get a deal done in February, it will be possible to hold a referendum in June, if we choose to do so. There is technically enough time.
"If the deal is not done in February, that would become much more difficult. Certainly, if it is not done in March, it would become impossible because of the timescales provided for in the Bill."
Mr Hammond said he expected European Council president Donald Tusk to provide proposals on the UK renegotiation for discussion at the Brussels summit within the next seven-to-10 days.
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."
Meanwhile, 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."
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.
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".
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.