Mr Yusuf appeared on Sky News, where it was put to him that there are people who will ask why should they trust what Reform UK says about Russia “given that one of their people took money from Russians”.
In response, Mr Yusuf said: “I think that would be an incredibly unreasonable position to take.”
Mr Yusuf, who was previously the chairman of Reform UK, added: “Nathan Gill, what he did was treasonous. It was horrific. It was awful. He’s now been dealt with by the authorities. He deserves the sentence that he’s gotten.
“But as you’ve just said, this is a guy which, far as we’re concerned, is ancient history.”
Mr Yusuf said he had never met Gill, and it was unreasonable to “besmirch” Reform UK, Nigel Farage and the party’s millions of supporters.
Gill, 52, from Llangefni, Anglesey, is thought to have received up to £40,000 to help pro-Russian politicians in Ukraine during his time as a Member of the European Parliament (MEP), when he accepted money from Oleg Voloshyn, 44, a man once described by the US government as a “pawn” of Russian secret services.
At the Old Bailey, Judge Mrs Justice Cheema-Grubb said Gill had abused his position and eroded “public confidence in democracy”.
Gill was a member of the UK Independence Party, who then followed Mr Farage to the Brexit Party and ultimately was – for a brief period before his fall from grace – Reform UK’s leader in Wales.