The European Union and Britain are closer than ever to an amicable divorce but the risk of a no-deal Brexit remains real, EU negotiator Michel Barnier warned Wednesday.

Briefing an EU and civil society meeting in Brussels as Britain swung into campaign mode for a snap general election under the shadow of Brexit, Barnier was cautiously upbeat.

The EU veteran hailed the "great new deal" he had reached this month with Prime Minister Boris Johnson's government, and noted that it had passed a preliminary vote in parliament.

Johnson has now called an election - hoping to win a majority to approve the accord - and EU leaders this week approved a three-month delay to Brexit day itself, he recalled. 

"So this is a key moment indeed," he told the European Economic and Social Committee.

But he warned that the risk of an economically damaging no-deal divorce persists if, for example, Johnson is not able to pass the agreement before the new deadline of January 31.

And even then, both sides will have to negotiate a tricky trade deal before the end of the year to govern future relations - or ask for an additional transition period.

"The risk of Brexit without a ratified deal still exists. It could happen at the end of January," he said.  

"It could also happen at the end of 2020 if we have a deal ratified before Jan 31 but do not manage to put in place a new agreement on our future relationship.

"The summer of 2020 will be a moment of truth. That will be the moment to decide whether or not to extend the transition period," he said.

The incoming president of the European Commission, Ursula von der Leyen has asked Barnier and his negotiating taskforce to continue to oversee the next stage of Brexit discussions.