It is "difficult to see" how a Brexit deal on the Irish border can be reached, the EU's chief negotiator has said.
Michel Barnier told reporters that "the current state" of UK thinking on the issue meant that a replacement for the backstop looks improbable – raising the prospect of a no-deal exit in October.
Boris Johnson has ruled out agreeing to the policy, which is supposed to prevent a hard border in Ireland – but the EU says it will not sign a deal without the backstop or an alternative that achieves the same effect.
The warning comes after the UK floated "concepts" to tackle the issue in meetings last week – which EU officials privately warned would not be enough to prevent checks and controls from reemerging between the Republic and Northern Ireland.
British officials were hoping for a breakthrough and change in EU thinking when Boris Johnson meets with EU council president Donald Tusk on the sideline of the UN general assembly in New York on Monday evening.
"Based on current UK thinking, it is difficult to see how we can arrive at a legally operative solution which fulfils all the objectives of the backstop," Mr Barnier said after meeting Germany's foreign minister in Berlin on Monday.