A Conservative candidate in this week's local elections was also a member of the United Kingdom Independence Party before resigning on Monday.
Melanie Heider, 38, is standing in Worcester's Arboretum ward.
A spokesman for the city's Conservative Party said Ms Heider had resigned her UKIP membership and remains a "loyal and committed" Tory.
Government chief whip Jacqui Smith, also Redditch MP, said Miss Heider had misled voters and should be sacked.
Politicians 'not trusted'
Marc Bayliss, Labour candidate for the Arboretum ward, said the situation was "bizarre".
He explained it was even stranger that Miss Heider had been given the chance to appear on a local radio show on Tuesday morning to give her views, but had declined to take part.
Miss Heider confirmed she had resigned from UKIP but has refused to make any further comment.
Ken Carpenter, the Lib Dem candidate, said the news did nothing to encourage people to trust politicians.
"The biggest comment we get on the doorsteps is people do not trust politicians," he said.
"Now we find somebody who is riding two horses at the same time."
Matt Edwards, agent for Worcester Conservatives, said: "Melanie Heider has resigned her membership of UKIP with immediate effect.
"She realised such membership is incompatible with membership of the Conservatives and has reaffirmed her commitment to the Conservative Party."
Five people are standing for election in the Arboretum ward, including another UKIP representative.
They are Marc Bayliss, Labour, Ken Carpenter, Liberal Democrat, Melanie Heider, Conservative, Jason Phillips, UKIP and Martin Sullivan, Green.