The Unite union official at the centre of the Grangemouth dispute has resigned from his job at the plant.

Stevie Deans was investigated by the company following his involvement in the row over the selection of a Labour candidate in the Falkirk constituency.

Grangemouth owner Ineos said in a statement: "The company has conducted a thorough investigation into Mr Deans' activities over the last 18 months and made Mr Deans aware of these findings last week.

"Mr Deans requested an additional five days prior to the final disciplinary hearing to allow him time to provide any further relevant information.

"The company was due to meet with Mr Deans again tomorrow but has now received his resignation."

Ineos threatened to close the petrochemical side of the business but changed its mind after the Unite union accepted changes to pay, pensions and other terms and conditions.

Unite said it will not comment until officials meet union members at Grangemouth.

The dispute between the union and Ineos was initially sparked by a row over the company's treatment of Mr Deans, who is the Falkirk party chairman and Unite's convenor at the Grangemouth plant.

Mr Deans and would-be Labour MP Karie Murphy were both suspended by the Labour Party as it investigated allegations of candidate-selection fixing in the Falkirk constituency earlier this year.

The investigation was closed last month after key witnesses withdrew evidence suggesting that they had been recruited to join Labour as part of a drive by Unite to cram the constituency party with supporters who would back its favoured candidate in a selection battle.

Both Mr Deans and Ms Murphy were reinstated as Labour members.

But Ineos proceeded with its own investigation into Mr Deans activities, prompting Unite members at the petrochemical site to ban overtime and opt to take other industrial action at the end of last month.

Meanwhile Ed Miliband was urged to consider reopening Labour's inquiry at the weekend, after the release a dossier of apparent new evidence.

Commenting on Mr Deans' resignation, Conservative Party chairman Grant Shapps said: "Stevie Deans' resignation is yet another reason why Ed Miliband must finally act. He must publish the internal Labour inquiry into Falkirk.

'Ed Miliband should also open a new inquiry: to investigate properly the initial allegations of selection rigging, and Unite's attempt to subvert the internal inquiry."

Scottish Conservative energy spokesman Murdo Fraser said: "This sorry saga has brutally exposed the unhealthy closeness of Labour and Unite.

"Johann Lamont must now undertake to cleanse the stables is she is to be taken seriously.

"The idea that the ego of one man could almost bring down an entire industry in Scotland is shameful, and Labour merely stood by and allowed it to happen."

The Sunday Times yesterday reported that it had seen emails suggesting that the letter retracting the witnesses' evidence in Labour's investigation was written by Unite officials and approved by Mr Deans.

The emails - sent from a company address - were reportedly obtained by lawyers working for Ineos during the dispute with Unite, as the firm sought to prove that Mr Deans was spending work time on political activities.

They reportedly included a draft of the retraction letter which was sent to Mr Deans for him to get it signed by Michael and Lorraine Kane, the witnesses who initially complained to the party against him and Ms Murphy.

The Sunday Times reported that a dossier of around 1,000 emails has been handed to police in Scotland, who were initially called in by Labour when allegations of irregularities emerged in July but determined that there was not enough evidence of wrongdoing to launch an investigation.

Commenting yesterday, a Unite spokesman said: ''Unite was the subject of entirely unjustified attacks in relation to the Labour parliamentary selection in Falkirk. Both the Labour Party and Police Scotland investigated the issue and found that neither the law nor the party's rules were broken by the union.

''The email exchanges, apparently leaked by an employer for its own purposes, do nothing to change that."

The Labour candidacy for the Falkirk constituency became available after the current MP Eric Joyce announced he would not stand again.