European Commission president Jean-Claude Juncker claims that Britain is "underestimating" the complexity of exit talks.
Speaking after EU leaders 'unanimously adopted' guidelines for the upcoming Brexit negotiations, Mr Juncker said discussions over EU citizens' rights would take "a huge amount of time".
He said the Commission had drawn up a proposed agreement "which could be adopted immediately if our British friends would be ready to sign it (but) that will probably not happen".
Mr Juncker added that "I have the impression sometimes that our British friends…underestimate the technical difficulties we have to face.
"The single question of citizens' rights is in fact a cortege of 25 questions that have to be solved".
Downing Street talks
He then hit out at Theresa May over her stance during talks at Downing Street, he said "Every time I was asking questions she told me to be patient and be ambitious," before adding "The same remarks have to be addressed to the UK".
He continued by saying that "Privately everything went well but we have a problem, the British want to leave the EU and it's not feasible that it can be done just like that."
Speaking alongside Mr Juncker, European Council president Donald Tusk called on the British Government to provide a "serious response" on the issue of EU citizens' rights.
He also hailed the "outstanding unity" of EU leaders who endorsed draft negotiating guidelines in just four minutes at today's summit.
Under the terms of the EU strategy, Britain must settle its so-called divorce bill and reach agreement on the rights of EU citizens living in the UK before negotiations can move forward to trade talks.
Additional reporting from IRN.