Boris Johnson has admitted Britain does get back some of the £350 million Brexit campaigners claim it sends to the EU every week.
But Mr Johnson told ITV News that EU officials decide how the money is spent and the "gross figure is the right figure".
The £350m claim is a central pillar of the "Leave" campaign and takes pole position on the side of the campaign's battle bus, which was unveiled in Cornwall on Wednesday morning.
Vote Leave say the £350m is "not well spent" and that "billions are lost to fraud and waste", but "Remain" campaigners have repeatedly discredited the figure, saying it is "misleading".
Video report by Tom Bradby:
In an interview with News at Ten presenter Tom Bradby, it was suggested to Mr Johnson that the true figure was more like £161m, something he said was "not in itself to be sneezed at".
"That figure (£350m) represents accurately the gross sum that is sent," the former Mayor of London said.
"Yes we do get some of it back, but we get it at the discretion of EU officials who decide how they're going to spend UK taxpayers' money in the UK."
Mr Johnson refused to admit the leave argument was misleading, adding: "If you take out the abatement and the money that comes back via Brussels, the figure is obviously lower.
"We think it's relevant to keep people focused on the global figure, because that is the figure over which we have no control."
Mr Johnson said he agreed with Michael Gove that Britain's future would lie outside the single market if it left the EU, but denied Britain would be poorer as a result.
"The single market costs UK business about £600m a week," he said.
The Uxbridge and South Ruislip MP said he was surprised by David Cameron's suggestion that Brexit could jeopardise peace in Europe, saying it is NATO that "keeps us strong and safe at home" and that the EU is "very much a distraction from that".
Mr Johnson's senior role within Vote Leave has prompted speculation about a bid for the Conservative Party leadership should his side win, but he denied there was any political element to the campaign, saying it was simply a "campaign for freedom".
The £350m claim has been criticised by the chairman of the UK Statistics Authority, Sir Andrew Dilnot, who said it was "potentially misleading".
Lucy Thomas, deputy director of Britain Stronger in Europe, previously said of the claim: "The fact that Vote Leave are reduced to using dodgy statistics speaks volumes about the appalling weakness of their argument."