Stephen Deans was previously put under disciplinary investigation by plant owners Ineos, claiming he used company time for union business. He had been expected to learn the fate of the disciplinary action today.
Unite had voted to strike over Deans' treatment and the threat of industrial action led to Ineos temporarily shutting down the facility last week. Ineos later announced the site's petrochemical plant would shut permanently with the loss of 800 jobs.
The company eventually reversed that decision after Unite agreed to implement changes to pay, pensions and conditions which Ineos said were necessary to ensure the survival of the petrochemical plant and the oil refinery.
In a statement issued last night by Unite, Deans, who had worked at the plant for 25 years, said if he stayed on "it would only prove to be a distraction, and would cast a shadow over what we all must hope will be a prosperous long-term future for the plant".
In his letter of resignation, Deans said management now treated him as the "enemy within". He said he decided to leave after Ineos leaked details of private emails to the Sunday Times in an effort to "further smear his reputation".
"The hurt to my family is unforgiveable and I will not allow management to take away my dignity and self-respect any further," Deans said.
Ineos released a statement confirming it had received the resignation: "The company has conducted a thorough investigation into Deans' activities over the last 18 months and made him aware of these findings last week. 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 Deans again tomorrow, but has now received his resignation."
