Conservatives have sought to turn up the heat in the Clacton by-election by offering voters – regardless of party colour – a chance to have their say on who they will pit against defector Douglas Carswell.

They announced plans for the for the US-style open primary – long hailed by their rebel former MP – amid Mr Carswell’s claims to have secured 150 pledges of support from either former or current Conservative Party members.

But Clacton Conservative Association chairman Simon Martin-Redman said just three members had tendered their resignation, and his association had in fact seen a “net gain” in members amid anger at Mr Carswell’s shock announcement last week.

Mr Carswell has long been a supporter of open primaries, and told the EADT that he wanted to see all the parties, including the UK Independence Party, use the system where it is not just party members choosing a candidate.

All voters in the constituency will be able to register for a public meeting to be held by the Tories on Thursday, September 11.

Candidates are still being interviewed by Conservative central office and a shortlist of between two and five people is expected to be drawn up and announced early next week.

Questioned on why he had not forced an open primary for his selection as a UKIP candidate in the by-election, Mr Carswell said: “I really don’t think anyone can accuse me of running shy from democratic accountability. I chose to resign from parliament and subject myself to a by-election.”

But he admitted that he would “feel a lot better” if the party’s general election candidate Roger Lord, who sensationally quit the party on Wednesday after being forced aside, was still with the party.

“He is a decent man. I wouldn’t say a word against him,” Mr Carswell added.

Letters are being sent to every registered voter in the Essex constituency by the Conservatives this week inviting them to register for the open primary next week.

It is expected that the extra cost of holding the open primary will be shared between the Conservative Party centrally and the association.

Mr Martin-Redman said: “(Nigel) Farage, when he got the Thanet nomination, stood up and said that people should have a democratic chance to elect people. Carswell, when he resigned, said he wanted a by-election so people could elect him. And of course they didn’t in his case. Roger Lord was the candidate. He was foisted upon the local association. So we thought we would give people a democratic voice and give people and open primary.”

Communities secretary Eric Pickles said: “By holding an open primary we are putting power into the hands of local people – letting them decide who the Conservative candidate for the by-election will be.”

The Conservatives have held just 22 open primaries since the last general election, despite a pledge in the Coalition Agreement to fund 200 all-postal primaries, targeted at seats which have not changed hands for many years.

Sarah Wollaston, the winner in Totnes in 2010, was the first to be selected through the method, although her selection was a postal vote, rather than at a public meeting.

She is now the chair of the influential Health Select Committee.

Witham MP Priti Patel was also selected through an open primary.

Ahead of her selection, David Cameron described primaries as an “exciting opportunity” to engage with the voters.