The British referendum on whether to stay in the European Union is now "more unpredictable than ever", according to European Council president Donald Tusk.
Mr Tusk also insisted he is still seeking to strike a deal on EU reform proposals next month to entice Britain to remain as a member of the 28-nation bloc.
He 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 summit of government leaders in February.
Mr Tusk said that "time is of the essence here". Talk in Britain has been that a referendum could come as early as this summer.