The President of the European Commission Jean-Claude Juncker has claimed the UK will "regret your decision" when the public comes to realise things “cannot remain as they are”.
Speaking in Strasbourg this morning, ahead of a vote to formally adopt a set of negotiating guidelines for the next phase of Brexit, Juncker called on Theresa May to "translate speeches into treaties", urging greater clarity and certainty from the government.
He added that any future relationship must be durable and not require continual renegotiation.
This is an apparent allusion to the position set out by May a fortnight ago, and reiterated by chancellor Philip Hammond, calling for a more outcome-based approach.
Hammond followed this up by calling for "fully equivalent regulatory outcomes", and agreeing to "bank our day one de facto equivalence".
Juncker, who is currently fighting off criticism of his move to promote Martin Selmayr under the radar, also called for the integrity of the Single Market to be maintained, adding there should be no undermining of social and environmental standards.
“As the clock counts down – with one year to go – it is now time to translate speeches to treaties," he added.
He also seized on the Irish border issue, saying the UK must put forward proposals on how to avoid a hard border, although he added it was a "European issue" not just an "Irish issue".
This prompted an angry response from UKIP MEP David Coburn, who retoted that it was a "British issue".
Michel Barnier spoke during the same session, telling MEPs: "We are open for business. It's the UK that is closing doors".
The EU's chief Brexit negotiator added: "All models of cooperation with third countries are on the table."
However he then went on to slam May's three basket approach, saying it was "rather surprising to think the EU could accept convergence, and at the same time retain the possibility for divergence when there is comparative advantage."
"It’s time to face up to the hard facts," he added in English.
On his controversial proposals regarding the Irish border, Barnier insisted it was just "a backstop", saying he would be prepared to replace it "with any other solution which is better".
Guy Verhofstadt, the Parliament's Brexit coordinator, also attacked May's speech last month for simply "repeating red lines" that have already been known for two years.
He told MEPs he was "quite confident" the UK would see the benefit of the association agreement proposal in tomorrow's resolution.