Zack Polanski has apologised for failing to pay the correct council tax while living on a London houseboat.
The Green Party leader had faced mounting questions over whether it was his primary residence.
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Government guidance says a person may be liable for council tax on a boat if it is their "sole or main" residence.
A party spokesperson said Mr Polanski had been living on the boat, moored in east London, "until relatively recently", which "came with its own unique practical circumstances and considerations".
"He has immediately taken steps to pay any council tax he may be found to owe," they added.
"Zack apologises sincerely for the unintentional mistake."
They said they would not comment on Mr Polanski's address "for security reasons", adding: "There have recently been two serious incidents which have been reported to the police and are under investigation."
Mr Polanski has come under heightened scrutiny since taking leadership of the Greens. The party's popularity has increased under his watch, and it enjoyed gains in last week's local elections.
The Greens previously told The Times he only stayed on the houseboat "occasionally" and rented a room at another address where council tax was included in the rent.
The newspaper had reported seeing an advert for the sale of the boat in which Mr Polanski's partner wrote: "We are moving to a house and so will sadly be leaving the gorgeous community behind."
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The Daily Mail separately reported that Mr Polanski was registered to vote at a building near the marina, which he is also said to have used as a mailing address.
Tax lawyer Dan Neidle, founder of Tax Policy Associates, analysed Mr Polanski's situation this week and wrote: "If the boat was in fact Mr Polanski's 'sole or main residence' then he and/or his partner should have registered for, and paid, council tax for those three years."
Mr Polanski's apology over the houseboat comes after he said sorry for sharing a post criticising the police response to the recent Golders Green terror attack.
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