European Union President Donald Tusk says the British referendum on whether to stay in the 28-nation bloc is now "more unpredictable than ever" and has insisted he is still seeking to strike a deal on EU reform proposals next month to entice Britain to remain a member.

Tusk told the EU legislature that despite progress on all the thorny issues that Prime Minister David Cameron wants dealt with before fully throwing his weight behind the campaign to stay in, "it will not be easy" to clinch a deal at a Feb 18-19 summit of government leaders.

Tusk said that "time is of the essence here." Talk in Britain has been that a referendum could come as early as this summer.