European Council head Donald Tusk said Wednesday there was "no guarantee" that EU leaders would reach a deal on keeping Britain in the bloc, just hours ahead of a make-or-break 'Brexit' summit.

"After my consultations in the last hours I have to state frankly -- there is still no guarantee that we will reach an agreement," said Tusk, who will chair the summit.

"We differ on some political issues and I am fully aware that it will be difficult to overcome them. Therefore I urge you to remain constructive."