Boyko Borisov, the Bulgarian prime minister, said he had so far seen nothing in the talks on Britain's exit in March 2019 to give him hope for a good outcome.
The comments will come as a blow to British Prime Minister Theresa May as her senior ministers embark on a flurry of speeches to outline Britain's ambitions for Brexit.
"I really hope that the talks on Brexit will take a turn, so that no damage will be incurred for anyone," Borisov told reporters in Sofia, where EU foreign ministers are gathering for talks.
"I'm not a big optimist. So far I haven't seen any progress than can really reassure me."
The EU wants to finish talks on the post-Brexit transition period by the end of March so negotiations on future ties with Britain can start, but has warned that deadline could be pushed back if there is no progress from London.
May's government is battling deep divisions between Brexit moderates and hardliners as it prepares for crucial talks with Brussels over the future relationship, which start in April.
On Wednesday British foreign minister Boris Johnson, who was a leading figure in the campaign to leave the EU, called for Britain to throw off EU rules and to view Brexit as an opportunity.
But Borisov urged Britain to reconsider its decision to leave, comparing Brexit to a family break-up.
"I truly hope the British come back to the European Union," he said. "One doesn't make the right decision every time so it's not embarrassing to change your mind."
"When people divorce, sometimes they get together again because of the children or other common things they share, so when we realise and they realise that it's hard for us and hard for them maybe we will find some smart compromise."
Earlier this month Lilyana Pavlova, the Bulgarian minister for the EU Presidency, warned May against plans to deny residency rights to Europeans arriving during the transition period.
Borisov said Britain and the EU negotiator Michel Barnier knew his country's position on the subject clearly, but said it was too early to talk of vetoing any agreement.
"Veto should be the last resort, only after all attempts have been made to reach a compromise," he said.
Eyewitness? Submit your stories now via social or:
Email: eyewitness@pulse.ng
![The woman was recorded, among others, while bathing [BI]](data:image/svg+xml;charset=utf8,%3Csvg%20xmlns%3D'http%3A%2F%2Fwww.w3.org%2F2000%2Fsvg'%20width%3D'300'%20height%3D'172'%20data-ring-placeholder%3D'1'%3E%3C%2Fsvg%3E "The woman was recorded, among others, while bathing [BI]")
