Shaun Woodward, the Tory MP who defected to Labour, scraped home to land the safe seat of St Helens South after an acrimonious contest that has soured the begining of the election campaign.

Although constituency members narrowly endorsed Mr Woodward, many said they resented having a candidate "parachuted" in following the decision of sitting MP Gerry Bermingham last week to retire.

One party official said there was a danger an Mr Woodward would now be challenged by an independent Labour candidate.

Activists were particularly angered that the four-strong short list imposed by Labour's ruling National Executive Committee did not include a local candidate.

Instead Mr Woodward was challenged by three councillors from outside the area. Marie Rimmer, a St Helens councillor for 20 years, was interviewed by the NEC but rejected.

Ann McCormack, St Helens vice-chairman for the Unison public service union, who was attending as an observer, said: "The majority of our members are Labour Party members and they have been denied their right to nominate someone. It is a stitch-up.".

One constituency member said: "This is a travesty. This town has managed very well without a Tory renegrade to represent it."

Labour had almost given up trying to find a seat for Mr Woodward, 42, to reward him for his defection 18 months ago. He is married to a Sainsbury heiress, has a butler and sends his children to public schools, making him hard to sell in a heartlands seat. Activists also strongly associate him with the Tories 1992 election campaign where he helped devise the "Labour's tax bombshell" advertising campaign.

On Sunday, the party privately defended the "parachuting" procedure.

"The one-member-one-vote system undoubtedly helps the local candidates in a selection contest. While these people, often councillors and union activists, have useful skills to offer, we need a wider range of experience on the back benches," said one party insider.

Under the informal arrangement, a veteran MP is encouraged to retire after the election has been called with the implicit promise of a peerage. The NEC can then impose a small shortlist on the constituency party often rigged to favour its preferred candidate, usually by barring any strong local figures.

David Clark, former cabinet office minister, last week decided to go, opening up the opportunity for a Downing Street-backed candidate to win a seat. David Miliband, head of the Downing Street Policy Unit, is expected to win his South Shields constituency.

There is speculation that one more senior Labour MP, possibly Jack Cunningham, may announce his retirement this week. A host of political advisers are still interested in finding a berth, including Derek Scott from the Downing Street policy unit, Simon Stevens, an adviser to Alan Milburn and his colleague Darren Murphy.

Ed Balls, chief economic adviser at the Treasury and Gordon Brown's right hand man, was hoping to get a seat in Yorkshire close to his wife Yvette Cooper's Pontefract constituency, but so far none has come available. Bill O'Brien, MP for the Normanton seat fancied by Mr Balls, has stamped on any suggestion he could be persuaded to reture.

Labour's selection of new MPs has been fraught during this parliament, with many constituency parties flexing their muscles and refusing to endorse anyone working for the government.

"It is a backlash against all the control freak stuff. To be seen as Millbank's favoured candidate is now the kiss of death," said one official.