The UK's EU renegotiation is not the top priority for EU leaders but the "ambition" is still for a February deal, the foreign secretary has said.
Philip Hammond said the talks "are number one on our Europe agenda" but the EU has "other issues on its plate".
It comes amid mounting speculation of a June vote after preparations for the referendum got under way on Tuesday.
Mr Hammond said a June date would be possible if a deal was agreed at next month's European Council summit.
But he stressed that it was more important to get the "right deal" rather than a "quick deal".
David Cameron is renegotiating the UK's relationship with the EU, ahead of the vote to be held by the end of 2017.
One of the main sticking points in the talks, appears to be UK proposals to restrict EU migrants' access to benefits for four years - with some countries warning it amounts to discrimination.
Addressing the Lords EU Select Committee, Mr Hammond said: "We have to recognise the fact the European Union has got a couple of other issues on its plate and although the British renegotiation is number one our European agenda I'm afraid it isn't number one on the European agenda of any of our partners, except perhaps for the Irish."
He added: "Unfortunately some of the other things are very much real-time challenges for the EU whereas this agenda is one which can be dealt with in slightly slower time.
"I think it would be inappropriate of us, and it would also be unwise of us, to try and push our agenda ahead of other things which the other EU member states will see as being pressingly urgent to discuss at the summit," he added.
But the foreign secretary said the UK government was "confident" there would be a "substantive discussion" on the renegotiation at the meeting.
Mr Hammond told the committee that if a deal was secured "it will be possible to hold a referendum in June, if we chose to do so".
"If the deal is not done in February that would become much more difficult. Certainly, if it's not done in March it would become impossible".
Adding to speculation of a June referendum, the government laid a motion on Tuesday's Commons order papers setting out some of the rules of the referendum, such as the length of the campaign, funding and the timing of the count.
It means the provisions of the European Union Referendum Act - legislation passed last year paving the way for the vote - will come into force from next month.
BBC assistant political editor Norman Smith said the move by the government was likely to face criticism from Eurosceptics given that Mr Cameron has not yet reached a deal with the EU over the reforms he is seeking.
It is thought the PM is anxious to have the rules approved by MPs as swiftly as possible - limiting the ability of people to delay the date of the referendum, he added.
Furthermore, the Electoral Commission has announced that registration for referendum campaigners will open on 1 February.
It applies to individuals and organisations planning to spend more than £10,000 promoting a certain outcome.
From this date, they must declare all donations and loans of more than £7,500. Campaigns must also check donations and loans are from eligible sources.
Any spending on campaigning prior to the start of the referendum period - which will be at least 10 weeks long - does not count towards a campaigner's spending limit.
Bob Posner, the Commission's director of party and electoral finance and legal counsel, 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."
David Cameron's four main aims for renegotiation
- Economic governance: Securing an explicit recognition that the euro is not the only currency of the European Union, to ensure countries outside the eurozone are not disadvantaged. The UK wants safeguards that it will not have to contribute to eurozone bailouts
- Competitiveness: Setting a target for the reduction of the "burden" of excessive regulation and extending the single market
- Immigration: Restricting access to in-work and out-of-work benefits to EU migrants. Specifically, ministers want to stop those coming to the UK from claiming certain benefits until they have been resident for four years
- Sovereignty: Allowing Britain to opt out from further political integration. Giving greater powers to national parliaments to block EU legislation
Referendum timeline: What will happen when?