The European Union's chief Brexit negotiator Michel Barnier has said some 25% of work remains to be done on a pact covering UK withdrawal from the EU, including crucial issues such as management of the Ireland/Northern Ireland border.
"In terms of what has been agreed so far, it's about 75%," Mr Barnier told France 2 television.
Even if Britain and the EU were working towards a British exit from the EU taking place in March 2019, risks of failure remained as long as outstanding topics such as Ireland were unresolved, he said.
"There are always difficulties, and risks of a failure,"said Mr Barnier.
Some in Britain want "what the English call cherry picking",he said, adding that the reply to that was "No way".
Britain and the EU are committed to keeping a free flow of people and goods over the Irish border without returning to checkpoints, which would be a reminder of the three decades of violence largely ended by the Good Friday Agreement of 1998.
However, no solution has yet been found for any customs checks needed after Brexit and a backstop plan could effectively isolate the Northern Ireland economy from mainland Britain.