LABOUR is facing the prospect of a damaging by-election in Falkirk after sitting MP Eric Joyce warned last night he would force a snap poll if his successor was selected by a political "fix".

The UK party last week suspended the selection process for a new candidate in Falkirk, amid fears the trade union Unite had enlisted dozens of new recruits in a bid to help its favoured contender Karie Murphy.

Party bosses are investigating claims that new members signed up in the town by the party's biggest affiliated trade union were pressurised into joining.

Labour is trying to select a new candidate for the 2015 General Election as a result of Joyce's conviction for assault last year.

However, Joyce last night told the Sunday Herald he would call an early by-election – instead of waiting to go at the next General Election – unless Labour sorted out the mess in the constituency.

"I intend to go all the way to the end of the term," he said. "However, this may change if I feel it is in the Labour Party's best interests to stop entryism by calling a by-election.

"If the Labour Party chooses to select someone who is obviously fixed by one vested interest, then there will be a by-election."

For the past two months, the contenders for the safe seat have been Karie Murphy, local councillor Linda Gow and communications expert Gregor Poyton. However, the contest has been marred by ugly allegations that Unite has been trying to aid Murphy, who works for Labour MP Tom Watson and is an ally of Unite general secretary Len McCluskey.

Unite helped fund a "consultative survey" on whether an all-woman shortlist should be used in Falkirk, an outcome that would benefit Murphy. The consultation was suspended amid fears the survey was not sent to every member of the local party, and that it included a biased question.

Eric Wilson, the UK party's constitutional officer, was then parachuted in to sort out the mess.

More controversially, a report produced by an internal grouping inside Unite claimed the union had recruited "well over 100 Unite members" to the local constituency.

This development alarmed local members, who believed it was an attempt to influence the selection vote. Wilson then contacted about 90 new members to inform them they could only vote in the contest if they filled out a direct-debit form.

However, the UK party took further action on Friday by suspending the entire selection process in Falkirk.

A party spokesperson said: "Concerns have been raised about membership recruitment which need to be investigated."

This newspaper can reveal one family complained to Labour weeks ago about the sign-up process. A second query was also received.

It is understood the investigation will consider whether any of the new members were coerced into joining.

Meanwhile, there is confusion about whether Murphy has withdrawn from the race.

Eight days ago, a senior party source said: "Karie Murphy is now not going to run."

However, the latest issue of the Falkirk Herald quoted Murphy saying: "I had initially intimated to my trade union and the national executive that I would be withdrawing from standing to dampen down negative press coverage. However, I have been overwhelmed by the support I have been shown and am reconsidering my position."

Asked about this quote, Murphy's lawyer at the leading London legal firm Mischon De Reya said: "We can confirm that our client was misquoted in the Falkirk Herald."

Earlier this month, Lord Mandelson criticised union influence on parliamentary selections. He said too many were "in the hands of one union at worst or a couple at best". He reportedly also said contests were being "orchestrated by a cabal of NEC (national executive committee) members" and that "this is not the new politics by any stretch of the imagination".

Unite has good links in Falkirk. Stephen Deans, Unite's chairman in Scotland, is also chairman of the town's constituency Labour Party.

An SNP spokesman said: "This whole saga has become a complete and utter farce from the Labour Party. They are taking the people of Falkirk for granted and are putting their own interests ahead of the people who really matter – the voters."

A Labour spokesman said: "An investigation is under way to ensure Labour Party members in Falkirk have an open and transparent selection process to select a new candidate."

A Unite spokesman said: "Unite is fully confident that in relation to Falkirk West, as in all seats where our members are active in the Labour Party, our conduct is correct and fully compliant with the rules of the party. Around half of all selections are all-women shortlists, which are determined by the Labour Party's NEC."