Carnage, the end of dreams and a return to the status quo: ask around in central Scotland about the Battle of Falkirk, and those who know say there's not much to celebrate. Now, more than 700 years since William Wallace's defeat by Edward I, a new battle is looming in Falkirk. Today the town will learn who will fight the next general election in the Labour Party's rose-red colours.

But rather than celebrate the likely selection of Karen Whitefield, the former MSP, as the Labour prospective parliamentary candidate (PPC), the divided constituency party will redouble their efforts to persuade the Labour leader, Ed Miliband, to publish a report currently languishing in his bottom drawer.

The secret report was written earlier this year by investigators probing allegations of vote-rigging involving the powerful Unite union seeking to win the PPC contest for their preferred candidate. Its non-publication continues to taint the party.

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By contrast, the local nationalist party in Falkirk are understandably upbeat about their chances next time. One SNP activist said, "Labour are often their own worst enemy. It doesn't get better than this." Both the Blairite right and the union-controlled left attempted to fast-track plans to get their preferred candidate chosen.

Although one contender from the party's right, the professional strategist Gregor Poynton, recruited 11 new members in a manner that challenged official guidelines, no one in authority took much notice.

However, the early activity from Mr Poynton – the husband of MP Gemma Doyle, a shadow ministerial colleague of Jim Murphy, the former Secretary of State for Scotland in the Brown government – did not escape the attention of Unite, which viewed Falkirk as a desirable seat.

A former member of Gordon Brown's cabinet told The Independent on Sunday: "The Blairites wanted another Eric Joyce. In fact there were advanced plans to fast-track the cut-off date, wrap up the contest and prevent the union-controlled left from getting their act together. Unite were reacting to a distorted contest that was already in play."

Mr Poynton boosted his chances with a clutch of new supporters. Unite sources in London say that, rather than 11 new sign-ups, "it was more like 30". Despite repeated efforts to contact Mr Poynton, he has remained silent on the issue.

By November last year, Unite's preferred candidate was Karie Murphy, a close friend of the union's general secretary, Len McCluskey, and then office manager for the MP Tom Watson in Westminster.

What Mr Poynton started, Unite intended to finish. A local union official, Stevie Deans, who had been a PPC contender before an all-women short list was imposed, is reported to have signed up around 150 new members in a manner that again sailed perilously close to breaching party rules.

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Mr Miliband, acting on the advice of officials in Scotland, ordered the investigation after a local councillor, Linda Gow, who also intended to join the contest, said some of her constituents had complained that they had been duped by Mr Deans.

In signing up most of the members added by Unite a scheme introduced by Tony Blair, allowing unions to initially pay for new members, was being used.

In March this year Mrs Gow helped two constituents complain about Unite's recruitment activities to senior officials in Edinburgh. She denies the complaints were driven by self-interest and were intended to destroy her rivals.

The complaints led to two months of investigation. It seems the resulting document focused on two periods and found that, inside the Westminster constituency boundaries, 131 people had been signed up by Unite between July and December 2012. Close to 100 members had joined the local party from December to May this year.

In June, with the report seen by a few people close to Mr Miliband, the Labour leader placed the constituency in "special measures", effectively taking control away from local activists and suspending the PPC contest. Anyone signed up after the announcement by Mr Joyce in March 2012 was excluded from the PPC vote.

Just weeks later, Karie Murphy and Stevie Deans were suspended from the Labour Party and Tom Watson resigned from his Shadow Cabinet post.

Without anyone in Falkirk, or in Unite, having seen the full report, Mr Miliband nevertheless described what was going on in as "machine politics" and "malpractice". Unite, however, were on the defensive, talking about smears and witch hunts, and how Mr Miliband's decision to review Labour's membership links with the unions was a "ruinous division".

Labour, based on the contents of the unpublished and largely unseen report, were on a collision course with the unions. Advisers close to Mr Miliband were said to be worried about an over-reliance on its central conclusion, and thought that eventually they would have to publish.

But there was a problem: party officials and their union equivalents now acknowledged that key players in the alleged scandal, including Mr Deans, Ms Murphy, Mrs Gow, the local family who sparked early complaints and others, were not contacted by investigators and were never shown the conclusions and central allegations of the report.

In damaging descriptions of how the entire probe was carried out, party sources suggested to the IoS that it was "a flawed exercise", that inexperienced interns may have been involved in an early draft where summary bullet points are not backed up by evidence in the main document and that, instead of a thorough examination of allegations of broken rules and potential criminal activity, it is "highly limited in what it looks at".

After an initial scoping exercise, police in Scotland found "insufficient evidence" for any further action. By September Mr Miliband was forced to abandon his earlier hard-line stance, re-instate those who had been suspended and accept that Unite had "done nothing wrong".

Mr Deans' union role in the neighbouring Grangemouth refinery strike and Unite's embarrassing retreat forced on it by the threat of closure, re-ignited the pressure on Mr Miliband to publish the Falkirk report. But he held out.

For some Scottish MPs the damage to the Labour Party's reputation and competency north of the border is self-inflicted and won't go away until the report, warts and all, is made public.

One MP commented: "Falkirk, and what happened in the race to replace Eric Joyce, could have happened anywhere across Scotland. This kind of mess could get dumped on other doorsteps if rules aren't enforced and no one knows who is in charge."