Theresa May's proposed new economic partnership with the EU "will not work", the head of the European Council has said.
Donald Tusk said the plans risked undermining the EU's single market.
He was speaking at the end of an EU summit in Salzburg where leaders of the 27 remaining member states discussed Brexit.
Mrs May said her proposals were the "only serious credible" way to avoid a hard border in the Northern Ireland.
Responding to Mr Tusk's remarks, she said there was "a lot of hard work to be done" to reach an agreement but added that the UK was preparing for no-deal.
The UK is due to leave the EU on 29 March 2019, and both sides are trying to reach a deal in time.
There is still no agreement on issues including how to avoid new checks on the Northern Ireland-Republic of Ireland border.
At a press conference, Mr Tusk said there were some "positive elements" in the UK's blueprint for future relations with the EU, which was agreed by ministers at Chequers in July.
Ms Sturgeon told the BBC that it would be completely reckless to leave the EU without establishing a future relationship.
She said that taking the UK off the "Brexit cliff edge" without an agreement "would be the most irresponsible thing any PM has done in a very, very long time".