A Brexiteer waves the Union flag as he demonstrates outside the Houses of Parliament in Westminster, central London on Thursday. Prime Minister Boris Johnson sought Thursday to blame the opposition Labour leader for his failure to deliver Brexit, as both men stepped up campaigning on the day Britain had been due to leave the EU. Photo: AFP

A top Brussels official has written a "love letter" to Britain expressing his grief at its decision to leave the European Union - and stressing it was always welcome back.

"I know you now. And I love you. For who you are and what you gave me. I'm like an old lover. I know your strengths and weaknesses," European Commission Vice President Frans Timmermans wrote in Thursday's edition of The Guardian newspaper.

"You have decided to leave. It breaks my heart, but I respect that decision," he added in a missive titled "My love letter to Britain: Family ties can never really be severed."

The Dutchman's letter concluded with the words, "We're not going away and you will always be welcome to come back."

The prospects of UK Prime Minister Boris Johnson reversing his decision to take Britain out of the EU on January 31 are close to nil.

Johnson won a thumping majority in this month's general election on a promise to "get Brexit done" after more than three years of indecision and delays.

Britons voted by a 52 to 48 percent margin to leave the other 27 EU member states in a 2016 referendum.

Most opinion polls indicate a slight majority in favor of remaining in the EU today.

But parties supporting a second Brexit referendum that could reverse the first one got drubbed in this month's vote.

Johnson has further indicated that he has no plans to extend the post-Brexit transition period beyond the end of next year.

Timmermans wrote that former UK prime minister David Cameron's decision to call the 2016 EU membership poll was a mistake.

"Was it necessary to force the issue? Not at all. But you did. And the sad thing is, I see it is hurting you," he wrote.

"And I fear more will follow," Timmermans cautioned.

The socialist Brussels veteran is the deputy to new European Commission President Ursula von der Leyen.