EU’s Juncker says he regrets not flagging Brexit leave camp’s 'lies' before vote

European Commission head Jean-Claude Juncker says he regrets not speaking out about Brexit lies told by the "Leave" side in the run-up to the UK referendum.

EU Commission President Jean-Claude Juncker gives a media statement after the meeting of the 27 European Heads of States and Governments in Brussels, Feb 2018. Source: AAP

The European Commission president thinks heeding former British prime minister David Cameron's request to stay silent while Brexit campaigners told "lies" before the 2016 referendum was the biggest mistake he made as EU chief executive.

At a Brussels news conference on Tuesday at which he laid out a defence of his record as president, Jean-Claude Juncker was asked to name the biggest error of a five-year mandate that ends in October.

He delivered a stinging assessment of Cameron over the Brexit vote.

UK Prime Minister David Cameron addresses an infrastructure business breakfast at the Barangaroo Delivery Authority in Sydney, 2014. Source: AAP

After faulting his personal response to a scandal dating to his time as Luxembourg premier, Juncker said the second mistake he made was to listen too carefully to Cameron.

"Because the then prime minister asked me not to interfere, not to intervene in the referendum campaign," Juncker said.

"It was a mistake not to intervene and not to interfere because we would have been the only ones to destroy the lies which were circulated around. I was wrong to be silent at an important moment."

Cameron angered fellow EU leaders by promising a referendum on Britain's membership of the bloc in a bid to prevent voters deserting his ruling Conservatives for the UK Independence Party.

Having secured some special concessions from the EU, he then campaigned to keep Britain in the EU in the June 2016 vote.

Juncker has said that he kept out of that British debate, partly because Cameron told him it would be counter-productive.

Many EU leaders were irritated, however, that Cameron's campaign made little mention of the special deal they had given him.

Cameron resigned after his defeat, handing over to Theresa May. She has negotiated terms for Britain's departure but has found them rejected by British MPs, delaying Brexit.

Asked if he now believed Britain might stay on, possibly as an awkward member, Juncker made light of the uncertainty, saying he had given up trying to guess British intentions.

Echoing a remark that he had made earlier in his presentation that "nobody understands England but everybody understands English", Juncker repeated a recent comment likening the British parliament to the enigmatic Sphinx of ancient Egypt.

"Either they will stay or they will leave," Juncker said. "If they stay they stay; if they leave they leave."