The Green Party said that a former parliamentary candidate whose name appeared on a British National Party contacts list is no longer a Green Party member.
Keith Bessant stood twice as a Green candidate in Cheltenham in the general elections of 2001 and 2005.
His name was one of 12,500 that appeared on a leaked list of BNP supporters that surfaced online this week.
A spokesman for the Greens said: "Mr Bessant left the Green Party for the BNP during a period of serious personal problems, the specifics of which it would not be appropriate to discuss. Our understanding is that he left the BNP after around two weeks."
He said that the party was unaware of anything that Mr Bessant had done during or before his Green membership that would have made him ineligible. But had he promoted the BNP's views during his Green membership, he would have "been disciplined and probably expelled".
Mr Bessant was unavailable for comment.
About the list, Gerry Gable, publisher of the anti-fascist magazine Searchlight, commented: "Although the list contains one serving police officer, what we are concerned about is that there are quite a large number of police officers who promptly left the police when the rules changed in 2004 - you can't be a police officer, or prison officer, and belong to the BNP."