Prospect National Secretary Mike Graham said: "The timing of this decision is unfortunate coming just days before the Nuclear Decommissioning Authority (NDA) begins the competition process for the new contract, due to be awarded in June 2014.

"On behalf of over 4,000 scientists, engineers and professionals involved in nuclear clean-up across the UK, we have obvious concerns over anything that could affect the stability of the company during the sale period and are seeking guarantees that measures will be taken to ensure any deal is conducted quickly and efficiently."

Graham said the union would be raising the issue with the Department of Energy and Climate Change to seek assurances that the sale of EnergySolutions would not de-rail the competition for a new parent management organisation for the Magnox and Research Sites Restoration Ltd sites.

"We have obvious concerns over how stable the company will be until a new parent management organisation is in place and therefore any transition arrangements must appear seamless to allow staff to focus on the safe and efficient clean up of sites."

In addition, said Graham, it is also vitally important that EnergySolutions' decision was not misconstrued as a vote of no confidence in the UK nuclear industry as a whole when, "the opposite is in fact the case.

"Setbacks across the UK's industry in recent months have been as a result of international pressures facing companies and their subsidiaries which require capital secured by divesting more profitable operations.

"We understand there is already interest in the purchase of the EnergySolutions' UK and European interests and we will be seeking private and confidential meetings with interested parties as soon as possible."