UNISON is not ruling out industrial action as a response to British Gas's decision to dismiss its entire 16,500-strong workforce and re-engage them on reduced terms.
The company has also served formal notice of its intention to end all recognition agreements with staff unions, of which UNISON is the largest.
As the union consults members on whether to move to an industrial action ballot, general secretary Dave Prentis has written to MPs at Westminster urging them to demonstrate their support for "these loyal and hardworking employees who are being treated in such an appalling fashion."
Most of those members work in British Gas offices across the UK, performing a range of functions largely dealing with customers. Many work part time.
Both British Gas and its parent company Centrica have suffered in recent years due to falling energy demand and other factors.
British Gas had achieved substantial cost reductions through job losses, site closures and other methods - all via negotiation and agreement with the respective trade unions.
More changes were expected this year, and UNISON was fully prepared to sit down and reach further agreement.
Instead, the company has issued its section 188 notice, giving advance notice of redundancy.
In a briefing document accompanying Mr Prentis's letter, UNISON calls the move "an unprecedented attack by the business on a loyal and productive workforce and a total u-turn on its former constructive relationship with its trade unions and employees.
"What Centrica is now trying to achieve, and its rationale for doing so, lacks integrity and in its current shape will provoke a serious dispute.
"It has already caused significant distress to employees and many are angry that the timing looks deliberate, with so many staff still working from home due to COVID conditions."
The briefing underlines UNISON's intention to negotiate "with integrity and the full support of our membership".
It adds: "We know that any industrial dispute will be damaging to all parties concerned, but ultimately if the changes the business is seeking to impose are not significantly improved, a dispute is a likely outcome, providing that this what members decide in a legal industrial action ballot."
Mr Prentis says the union was "dismayed" at the treatment of loyal employees who, during the COVID-19 pandemic, were deemed critical workers who continued to support customers and ensure their energy supply was unaffected.
"The law allowing the dismissal of a total workforce with a view to forcing them to accept reduced terms is an unacceptable practice in the best of times, but doing so in the midst of the current pandemic is utterly disgraceful."
Our energy service group supports members facing issues at work and aims to improve pay and conditions. We also campaign on issues which affect the wider community such as fuel poverty, sustainable and renewable supply and environmental safeguards.