Former Irish premier and EU ambassador John Bruton has launched a blistering broadside on Boris Johnson.
Mr Bruton said the Foreign Secretary had "criminally misled" the UK over the consequences of leaving the EU, before rowing back and tempering his remarks.
"Boris Johnson attempted to simplify the Brexit issue by saying we can have our cake and eat it," he told a parliamentary committee in Dublin.
"It was a memorable phrase and it was criminally wrong.
"It criminally misled - I shouldn't say criminally, it is not criminally - but it irresponsibly misled the people of Britain."
The ex-Taoiseach, who was EU ambassador to the US between 2004 and 2009, said the UK was thrust into a predicament because it did not try to understand the complexities involved.
Mr Bruton said smaller countries like Ireland were forced to get to grips with how the EU operates for their own interests.
"The UK felt until now that they don't have to understand the EU, that they would make a decision to leave and that the consequences were going to be light," he told senators on a special Brexit committee.
"I think they are going to find that the consequences will not be light - and that is the consequences they are paying for this search for too much simplicity."
Mr Bruton also suggested the UK might yet change its mind on leaving the EU, once it fully realises the alternative.
"While I believe it may seem impossibly optimistic today, I believe conditions can be envisaged in which, eventually, the UK voters might either decide, either not to leave the EU at all, or to decide, after it has left, to rejoin," he said.
Mr Bruton said Ireland should work "to keep that possibility alive".
The current hard Brexit proposed by Theresa May would do incalculable damage to Ireland "politically, emotionally and economically", he warned.
"We cannot simply wait for this to happen," he added.
"While seeking to mitigate the effects of Mrs May's chosen hard Brexit, we must also do everything we can to ensure either that, at the end of the day, there is no Brexit."