Bernard Kouchner, who has served in governments as part of the Socialist Party and an independent, was stopped in his tracks by radio host Justin Webb.
The politician was speaking on BBC Radio 4’s Today programme about the Prime Minister’s chances of maintaining the “greatest possible access” to the single market when Britain leaves the EU.
He was also asked about Mrs May’s chances of securing a free-trade deal with the European Union following her interview with French newspaper Le Figaro.
Mr Kouchner replied: “You know, I’m a great admirer of British democracy and very close to all the Brits… so I was really disappointed the way you did (sic), the way the referendum was unprepared.”
Bernard Kouchner said he was "disappointed" by Brexit
I think that her goal is to maintain the same relation with the big market, the European market – it will not be possible
But Webb interrupted and told the politician: “Yeah but we've done that now just address if you would, that point that she makes to directly to the French people in Le Figaro this morning.
“This should be the deal.”
Mr Kouchner then said Mrs May’s Brexit vision “will not be possible”.
“This morning there is a letter coming from Mme May and she said [what] she wants, she's leaving the Union but she's not leaving Europe – OK this is a word (sic).
The Top 10 UK Trade Prospects
Thu, February 16, 2017
What are the Top 10 trade partner prospects for the UK moving forward?
Get Quotes on Home Insurance
1 of 11
Bernard Kouchner said Marine Le Pen was the most "stable" French candidate
“I think that her goal is to maintain the same relation with the big market, the European market – it will not be possible.
“There is an example, the Swiss, they refuse to come to the European Union and they are maintaining some particular commercial relation – I don't know if that's the goal.”
Speaking about Marine Le Pen’s chances at the French elections, he admitted she was the only “stable” candidate.
“We are crossing troubled waters, at all the levels of this election,” he said. “It's very particular, the only one where the leaders, all of them, are not very precisely politically defined.
“Mr Fillon is crossing also a very personally difficult time, Mlle Le Pen, if I may say so, [is] the only stable candidate.”
Webb said it was “extraordinary in itself” to which Mr Kouchner replied: “Tes it is, it is dangerous also.
“She's going ahead, walking peacefully, the others are agitated, looking for their audience.”