Shaun Woodward, the Tory defector selected to fight the safe Labour seat of St Helen's South, is to face a challenge from a disaffected member of the local Labour Party.

Neil Thompson will leave the party and run against Woodward as a Socialist Alliance candidate.

Thompson said he was "very, very disillusioned with the way this individual has been brought into our constituency to try to represent the people of St Helens".

Woodward, who ran the Tories' election campaign in 1992 but defected to Labour in 1999, was selected to fight the seat as a Labour candidate on Sunday with a majority of just four in a poll of local party members.

The vacancy arose when sitting MP Gerry Bermingham decided at the eleventh hour not to seek re-election.

The move provoked anger amongst Labour members who said that four local candidates had been excluded when Labour's Millbank HQ drew up the shortlist.

Woodward, a millionaire who has a butler visited the seat for the first time last Wednesday.

Critics say he was parachuted into the seat by senior Labour figures see him as good ammunition against his former party.

The Socialist Alliance launched its election manifesto, "People before profit", outside Labour's Millbank HQ yesterday, minutes before Tony Blair was due to launch his party's own manifesto "Ambitions for Britain."

At the launch, the prominent left-winger Dave Nellist attacked new Labour. "What Tony Blair now does with a smile, Margaret Thatcher used to do with a snarl," he said.