The European Council president said, going on media reports, that the UK was still intent on “cherry picking” its future relationship with the EU.
Theresa May will deliver a long-awaited speech setting out her vision for Britain’s ambitions on Friday.
The Prime Minister cleared the way for the speech by securing agreement from senior ministers at a meeting at Chequers on Thursday on key features of the future relationship.
The Government will pursue a policy putting Britain outside a customs union with the EU but matching Brussels rules in certain sectors in an attempt to achieve “frictionless” trade.
At a press conference in Brussels, Mr Tusk said: “I am glad that the UK government seems to be moving towards a more detailed position.
“However, if the media reports are correct I am afraid that the UK position today is based on pure illusion.
“It looks like the cake philosophy is still alive. From the very start it has been a key principle of the EU 27 that there can be no cherry-picking and no single market a la carte.
“This is, and will continue to be, a key principle, I have no doubt.”
Mr Tusk will travel to London for talks with Mrs May on Thursday, the day before the Prime Minister’s Brexit speech.
The Prime Minister’s official spokesman described the eight-hour Chequers meeting as “very positive… and a step forward, agreeing the basis of the Prime Minister’s speech on our future relationship”.
The meeting’s conclusions will be discussed by the full Cabinet ahead of Mrs May’s speech – probably at its regular Downing Street meeting on Tuesday.
Health Secretary Jeremy Hunt, who did not attend the Chequers talks, said the Brexit sub-committee agreed that Britain must not be part of a customs union as it should have the right to strike free trade deals with other countries.
He claimed “frictionless” trade was still possible without one.
He told BBC Radio 4’s Today programme: “Customs union is one way of getting frictionless trade but it’s not the only way, and what we’re saying is we want to achieve frictionless trade by agreement between two sovereign bodies – the United Kingdom and European Union.”
He added: “You have divergent views on a big issue like Brexit, as you would expect, but the central common understanding is that there will be areas and sectors of industry where we agree to align our regulations with European regulations.”