UKIP is considering suing two London Assembly members who defected to Kilroy-Silk's new Veritas party.
Damian Hockney, now Veritas deputy leader, and Peter Hulme-Cross were elected in 2004 on the list system.
The party argues the pair should give up their seats as they won them as UKIP representatives, not as individuals.
Mr Hockney said the law was clear that those elected on a list who quit their party should keep their seats.
UKIP chairman Petrina Holdsworth urged the men to step down from the GLA in a letter.
In the letter, Mrs Holdsworth said: "In the circumstances it is risible that you can put yourself forward as supporters of a party which has declared that it stands for the truth when all those who hold elected office in your party gained that office through UKIP votes on a party-list basis.
"The party has taken legal advice and it is clear that we could take legal proceedings against you which could result in the return of our seats and / or damages against you .
"We would however like you to be given an opportunity to reflect on what you have done, to restore your own credibility with the electorate and return the seats to the party which won them fair and square at the last election."
Press Association, Thursday 24 February 2005