David Cameron saw off rebellious activists last night after party members in Norfolk voted to endorse a candidate accused of covering up an affair with a Tory MP.

The victory for Mr Cameron — by almost four to one in a secret ballot — means that Liz Truss remains the Tory candidate for South West Norfolk and indicates that a majority of grassroots Conservatives are prepared to back the leader’s modernising project in order to return to power.

Mr Cameron put his authority on the line at the meeting with a note from party HQ admitting that mistakes had been made in its handling of the selection process. The letter, from John Maples, the deputy chairman in charge of candidates, urged the 169 local members gathered in Swaffham Assembly Rooms to support Ms Truss.

The local party was deciding whether to deselect her after reports about her relationship in 2005 with Mark Field, the City of London MP, surfaced the day after her original selection in October. Although some details of the affair were in the public domain, the newspaper reports were the first time that many in the local party had been made aware of it. Several felt that Tory high command’s failure to mention the fact in briefings amounted to a breach of trust.

The final result was a 132-37 vote in Ms Truss’s favour. Afterwards, she said that she would work with everyone to defeat the Labour Party. “I have been sorry about the way things have turned out . . . we are hoping to move forward,” she told BBC Look East. “I want to work with everybody in the local party. All the people who supported me and those who didn’t.”

Shirley Matthews, a supporter, said she was delighted, adding that most of Ms Truss’s opponents were angry with the procedure rather than her.

Mr Cameron vested considerable personal authority in her survival, with the party sending Baroness Shephard of Northwold, a former Norfolk MP, with Ms Truss to the meeting. But the row has brought to the surface deeper tensions between Tory high command and local parties. It also raised questions about the party’s attitude towards female candidates, forcing them to justify their private lives in a way that many believe would not happen to a male counterpart.

The South West Norfolk association had accused party HQ of bullying and bribery in an attempt to keep Ms Truss, who does not live in Norfolk, in place. Senior Tories attacked “idiots” and “turnip-head” extremists in the local association, saying their sexism made the Taleban look moderate.

Mr Cameron confronted Sir Jeremy Bagge, a former High Sheriff of Norfolk and leading rebel, as well as vowing to impose an all-female shortlist if Ms Truss were thrown out. Afterwards Sir Jeremy said: “I am not proud to be a Conservative just at the moment. Central Office has deceived and betrayed us. These are strong words but that’s how I feel.”

Adding to the toxic atmosphere were briefings that one of Ms Truss’s opponents, a local councillor, Cliff Jordan, was “hardly squeaky clean” because three years ago he was ordered to repay £17,000 of falsely claimed invalidity benefits. The association was further dismayed after the Eastern Daily Press reported that Ms Truss was selected shortly before the 2005 election to fight Calder Valley, where a previous candidate had been unseated over allegations of an affair. Unbeknown to Tories there, the relationship with Mr Field was under way at that point.

One source said that the Tory leadership had gone to extreme lengths to secure last night’s vote, repeatedly calling association members to persuade them to support Ms Truss. “Some people were threatened, told ‘If you deselect, you will lose us the general election’. Others [were] offered incentives. There’s talk they’ve offered them OBEs.”

Ms Truss, who comes from Leeds and is a member of Greenwich Council in London, said that she planned to move to the area. “I am already renting a place in Swaffham,” she said. “I am going to buy a place in due course.”

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