Officers found “no evidence of any criminality” following an inquiry triggered by a complaint about the activities of Stevie Deans, who was a full-time Unite official at the Grangemouth refinery.
Mr Deans left his job as a Grangemouth employeeafter the site owner, Ineos, claimed he used company time for political campaigning work that involved sending e-mails linked to his then role as chair of Falkirk Labour Party.
However, Police Scotland said there was no evidence of criminal conduct after a number of the alleged e-mails were sent to the force.
A Police Scotland spokesman said: “Following information received alleging misconduct by a member of staff at the Grangemouth refinery, a Police Scotland inquiry was undertaken. This inquiry has now concluded and there is no evidence of any criminality.”
Unite was accused of trying to sign up members to ensure its favoured candidate in Falkirk, Karie Murphy, was selected to contest the safe Labour seat at the next general election.
The union, which has always denied doing anything wrong, yesterday claimed the allegations about the conduct of Mr Deans at Grangemouth had wasted police time.
Mr Deans did not seek re-election as chairman of Labour’s constituency party in Falkirk, after he was earlier suspended during the row, along with Ms Murphy.
Unite general-secretary Len McCluskey insisted Mr Deans had been “smeared” by the allegations that included claims that union members were signed up to Labour in Falkirk without their knowledge.
Mr McCluskey said: “It is shameful that the police’s time has been wasted by vexatious complaints and their attentions diverted from catching real criminals and solving real crimes.
“Stevie Deans is a decent and honourable man who has been smeared and hounded with a callous disregard for him and his family by those who should know better.
“The anti-union hysteria whipped up by certain sections of the media and their friends to pursue a spiteful agenda has been shocking. Their witch-hunt has been exposed to be without foundation and a lie.
“Unite has been vindicated in consistently saying that no wrongdoing or criminality has taken place and welcome Police Scotland’s conclusion.”
Disciplinary proceedings by Ineos against Mr Deans led to a vote for strike action at Grangemouth and a temporary shutdown of the plant last year.
That started a sequence of events that led to Ineos announcing it would close the plant before performing a dramatic U-turn.
The company defended its action against Mr Deans, who is now employed by Unite as a union organiser in Scotland.
A spokesman for Ineos said: “The Ineos investigation of Mr Deans was related to the misuse of Ineos procedures and systems.
“Mr Deans resigned prior to the final stage of the disciplinary process. The e-mail cache was referred to the police and the information commissioner, based on legal advice to protect the company.”
Labour was forced to select a candidate to replace MP Eric Joyce, who resigned from the party after he was convicted of assaulting Tory politicians in a Commons bar in 2012.
However, the selection process has been dogged by controversy, with Labour officials conducting an internal inquiry into the allegations of vote-rigging.
Labour last night insisted that the issues had been resolved in Falkirk, where former MSP Karen Whitefield has been selected as its election candidate.
A Scottish Labour spokeswoman said: “With a strong candidate now in place, our focus is firmly on putting forward the case for Labour to the Falkirk electorate so they get the committed Member of Parliament they deserve.”