Peter Davies, the mayor of Doncaster, has resigned from the English Democrats claiming there has been a big influx of new members joining from the British National Party.
Mr Davies, who was elected to the post in 2009, had planned to stand for a second term at May's local elections.
He will now continue in office as an independent until the poll.
Party chairman Robin Tilbrook said he was saddened at the development but denied the party had been infiltrated.
The BBC's Yorkshire political editor Len Tingle said it had been known for some time that Mr Davies - whose son is a Conservative MP - was uncomfortable with his party's current direction.
(A thorn in the side of a Conservative MP perhaps? Did his son persuade him into jacking the English Democrats because of the damage it could do to his career?)
Mr Tilbrook claimed that it was not certain that the English Democrats would have selected Mr Davies to stand again as he had not followed party policy during his term of office in promoting its main aim to create an English Parliament.
He strongly denied that his party had been infiltrated by far-right elements and pointed out that any new member has to sign a pledge confirming their opposition to racism.
Mr Davies campaigned on behalf of the UK Independence Party in the 1990s before joining the English Democrats in 2005.
The English Democrats, whose only electoral success has been in Doncaster's mayoral elections, say they will choose another candidate at a selection meeting later this month. The party secured 0.2% of the total vote in the 2010 general election.
Residents of Doncaster voted to keep the mayoral post in a referendum last year, having originally approved it in another public vote in 2001.