Mayor of London Boris Johnson is right in stating the European Union has a democratic deficit, former Greek finance minister Yanis Varoufakis has said.

While Varoufakis wants the UK to stay in the EU, he said that when Johnson declared support for Brexit "he said, quite correctly, that Brussels is suffering from a serious lack of democratic controls. He was utterly right".

The former Greek minister wants the UK to stay in the EU as he thinks Europe is sliding into an "abyss" and wants the continent to pull together to solve problems and push for democratic reform from within the 28-member bloc.

Speaking to CNBC, the economist added that Prime Minister David Cameron and chancellor George Osborne face their biggest opposition in the EU referendum from members of their own party, notably Johnson, justice secretary Michael Gove and former Tory leader Michael Howard.

Read more: Yanis Varoufakis is going to advise the Labour party

"They have a very strong intellectual case against the European Union," he said. "I am campaigning in favour of Britain staying in, but the intellectual strength of the Johnson opposition is very good."

So when George Osborne comes out and pokes fun at me, obviously trying to luxuriate in the fact that I'm a defeated finance minister. Yes, I am a defeated finance minister. But in the hands of whom? Of an iron-clad European Union that decided to asphyxiate us using bank closures in order to impose upon us another extend and pretend bailout. The British people know that. Does George Osborne really seriously believe that by mocking me he is doing himself any favours in his intellectual clash? I don't think he is doing himself any intellectual favours.

Varoufakis' comments come after the chancellor made fun of Labour for comments by Jeremy Corbyn confirming that the former finance minister was advising the opposition. "Varoufakis was recruited because Chairman Mao was dead and Mickey Mouse was busy," Osborne said.

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But Varoufakis said that he spoke to people from many political parties, including Lord Norman Lamont of the Conservative party and Caroline Lucas of the Green party.

On a lighter note, Varoufakis retorted to another of Osborne's claims, when the Conservative minister said that shadow chancellor John McDonnell and Varoufakis share a similarity – that they've lost their marbles.

The Greek's reply: "Well, our marbles were stolen, George."