Hundreds of workers at the Sellafield nuclear waste plant are being put at risk of contracting coronavirus, a union has claimed.
Unite is threatening to take legal action against Mitie, the company employing staff such as cleaners and security workers at the Cumbrian site.
It said the firm was "failing" to follow government guidelines.
Mitie denied the allegations and said the safety of its workers was its "top priority".
Unite said it had asked for risk assessments to be carried out for each building people were working in but had not received a response.
It also requested Mitie arranged buses to transport people to work to enable them to stay 2m (6.5ft) apart rather than car share.
In a letter seen by BBC Cumbria, Unite alleges Mitie is "failing to operate a safe system of work" and is putting staff in "imminent and serious danger".
It adds the "danger has the potential to be extremely serious and in some cases deadly" and called on the firm to "act immediately" or it would consider applying for an injunction.
About 600 Mitie workers are based at Sellafield, including catering, pest control and engineering staff.
The firm said a number of its staff are self-isolating.
Mitie added it had completed enhanced risk assessments for every building with daily checks to identify new issues and from Wednesday it will also provide free buses.
"All health and safety processes on site are in line with the latest guidance from the UK government and are evaluated and updated as required on a daily basis," the company said.
"This includes implementing new policies - such as social distancing, an increase in sanitisation cleans, additional personal protective equipment and adjustments to transport arrangements - as well as extensive communication to all employees to ensure procedures are followed."