Boyko Borisov said it was his “sentiment” that the divorce talks will crash and burn, adding that anyone who believes the EU can easily brush aside such a setback is “overestimating” the club.
His remarks significantly diverge from those made by other senior European leaders, who have always insisted that there will be a deal but that the bloc is prepared for any eventuality.
The colourful politician is a former professional footballer and blackbelt karate coach who once bear hugged David Cameron so hard the ex-British PM thought he would die.
In a series of unguarded remarks, Mr Borisov warned his fellow EU leaders of the dire consequences they face if they do not manage to achieve a good trade settlement with the UK.
He told Euractiv: “Regrettably, this possibility is more and more mentioned, that there would be no agreement. I am not saying it on behalf of the EU or of our presidency, and I don’t want to be misunderstood.
“But this is my sentiment. I hope that we will keep our good relations. Let optimism prevail and let there be a good outcome. And the best is if they stay in the EU.”
Mr Borissov praised the bloc’s chief negotiator, Michel Barnier, as an “extraordinary diplomat” and said he “truly hopes” he will be able to sign off on sufficient progress next month.
And he appeared to divulge the contents of private conversations with other leaders when he said that no European leader seriously believes the EU is ready for the chaos of a no deal Brexit.
He said: “There is hardly anyone in the EU, except in the UK, who does not realise that Brexit would be bad for the EU. The exit of one of the most powerful economies is not a good thing.
“If I were to say we are ready, it would be overestimating ourselves. If you ask whatever prime minister or chancellor here, they will tell you they are not ready.”
The Bulgarian PM said he wanted to see the “extraordinary” cooperation between Britain and the EU on a number of issues, including security and policing, kept “at the same level” after Brexit.
On talk on a new security and defence pact, he said: “Not only am I not against this, it would be wonderful for Bulgaria and for security in Europe. The same goes for the numerous UK citizens in Bulgaria.”
Mr Borissov’s remarks are by far the most outspoken of any European leader and are likely to irritate some of his colleagues, coming just weeks before December’s crunch summit.
Both sides want to achieve sufficient progress and move onto trade talks at the meeting, but have warned the divorce could still be help up by outstanding disagreements on cash and Ireland.
Dublin has significantly ratcheted up its rhetoric on the border issue in recent weeks as it calls for the UK to guarantee there will be no regulatory divergence on the island of Ireland.
However, such a solution would effectively mean moving the UK border into the middle of the Irish Sea – a scenario which is unacceptable to the unionist DUP preparing up Mrs May’s Government.