Foreign Secretary Boris Johnson's vision of the UK leaving the European Union but still trading freely with member states is an option that does not exist and is "intellectually impossible", said Dutch Finance Minister Jeroen Dijsselbloem.
Speaking to the BBC's Newsnight, Dijsselbloem accused Johnson of putting forward options for the UK that "are really not available."
The finance minister, who is also the president of the eurozone's Eurogroup, said: "He's saying things that are intellectually impossible, politically unavailable, so I think he's not offering the British people a fair view of what is available and what can be achieved in these negotiations."
He insisted that while firms operating in the UK currently have full access to European markets "without any hindrance or customs duties," some of that "will disappear."
The EU customs union allows goods to move between member states without tariffs. Member states are required to apply tariffs on goods imported from outside the union.
Dijsselbloem warned that both the UK and the European Union member state economies would be in "a worse situation" in a post-Brexit world.
"There is no win-win situation. It's going to be a lose-lose situation and in the best case, if we set aside all emotions and try to reach an agreement that is least damaging to both of us we can minimise the damages," said Dijsselbloem.
"We can do our best to minimise damages but it's going to be a step back and that is what Boris Johnson should start talking about."
Johnson, in an interview with a Czech newspaper, said that the UK was likely to leave the EU customs union but that it still wanted to trade freely with member states. Downing Street has said that no decision on membership of the customs union has been made.
There is concern that Prime Minister Theresa May's administration does not seem to have any clear strategy for Brexit negotiations after a memo compiled by consultancy firm Deloitte and leaked to The Times indicated divisions within the Cabinet.
A Number 10 spokeswoman has already dismissed the memo saying that it was "unsolicited" and that Deloitte did not have access to Number 10 for the report.