The union, which represents 130 members at Wherstead Park and 2,000 in E.ON across the UK, said that just prior to yesterday's announcement the company attempted to introduce new redundancy terms, which in many cases reduce the previous entitlements of staff now under threat of losing their jobs.
E.ON claims the Wherstead Park location is too distant from its core business and too expensive to maintain.
Prospect negotiator Richard Hardy said: "Our members are angry and disappointed about both these announcements. It adds injury to insult to cut your redundancy terms one week and announce job losses the next.
"If the Wherstead workers had been made redundant last week they would have received a total of £2.5m in compensation. Under the new terms this would only amount to £1.9m. This cynical handling of staff is disgusting and not the way that you would expect a major multi-national company to behave."
During the upcoming 90-day consultation process, he said, the union will have two distinct aims; to campaign to keep the Wherstead site open and to protect and improve severance terms for all E.ON members.
Prospect, which represents engineers, specialists and professional staff in the electricity industry, will now undertake a study of the company's business case and consult members and other stakeholders.
E.ON employs over 10,000 people in UK businesses such as Powergen, Ratcliffe Power Station and Central Networks.