Alexander Fiuza, of eurosceptic group Get Britain Out, insisted the European Union’s “illegal” demands for a high bill for when the UK leaves the bloc, were unjustified.
The research executive said Britain should pay less than the £36 billion figure, which British officials have reportedly conceded they are prepared to pay.
But Mr Fiuza hit back and said there were “contributions” the EU owed Britain.
Speaking on RT, he said: “What we have to do is be honest. When the EU talks about the Brexit bill, they’re not talking about anything remotely legal.
“There are obligations that we have to them. Things like our remaining budgetary contributions, things like contributions to projects we’ve been heavily involved in.
“But they’re asking us to pay for bureaucrats’ pensions, they’re asking us to pay for new ventures in military and such which will go on years after we’ve left and which have barely started now.”
Put to him that the UK has to pay a bill of some kind, Mr Fiuza highlighted the UK’s stake in European projects.
He said: “Yes, there are some obligations we have to pay and they’re are also entitlements they have to pay for us.
“For example, we own 16 per cent of the European Investment Bank and we have stakes in public works across the EU.
“So what it ends up with, including their contributions back to us, which we’re legally obliged to give in due course, maybe altogether less than the EU’s €100 billion (£92 billion) or even the compromised £36 billion.”
The so-called Brexit bill has been repeatedly disputed in recent weeks.
Meanwhile, Nigel Farage has slammed Angela Merkel, saying she “can go jump” after she said Britain had an “obligation” to pay the Brexit bill.
MEP and former Ukip leader Nigel Farage has slammed the German Chancellor saying she “can go jump” after she waded into the debate about the Brexit bill, saying Britain had an “obligation” to pay.
Mr Farage told Express.co.uk: “She can go jump. To ask the UK to pay a Brexit bill without agreement on a free trade deal is tantamount to blackmail.
“Their make-believe Brexit bill has not legal basis whatsoever and the EU knows it.”
The German Chancellor, who has no official role within the bloc, decided to weigh into the ongoing debate in her weekly podcast.
Acting as a mouthpiece for the European Union, Mrs Merkel warned the disputed bill will need to be settled before Britain leaves the bloc.