The far-right British National Party has increased its council seats to 17 with the defection of a Yorkshire Conservative councillor.
Geoffrey Wallace, a member of Calderdale Council, West Yorkshire, said he had crossed to the BNP because the Tories were "strangled by political correctness".
He said: "The British National Party is the only political party which will stand up to the European Union and defend British culture."
His defection - the first time the BNP has won over a serving councillor from another party - brings its number of seats on the council to three. It now has 17 in England, with the majority in Lancashire and Yorkshire.
Mr Wallace was condemned by Liam Fox, the Conservative party chairman, and faced protests from union leaders and MPs. Dr Fox said: "People whose instincts and beliefs lead them to join the BNP have no place in the Conservative Party. We do not regret the departure."
Sir Donald Thompson, the chairman of the Calder Valley Conservative Association, dismissed the significance of the defection. He said: "The BNP is the party of small change and bar-room politics."
Brendan Barber, the general secretary of the TUC, said: "The Conservative voters who elected this man to represent their views will expect him to resign."
Mr Wallace, a building worker, who has held the Illingworth ward for four years after winning it by 200 votes, said: "It is a bold move, but it's a move I had to take. I do not like cheating anybody with regards to standing by my principles, but most of all I don't like cheating myself."