Companies are being threatened with prosecution if they do not make premises "COVID secure" for returning workers.
The warning comes as people in England are told to go back to work from Wednesday if they cannot do their job from home – such as builders and manufacturers.
Ministers have stressed firms should ensure social distancing stays in place as the lockdown is slowly eased over the next few months to stop the spread of coronavirus, which has already killed 32,692 people in the UK.
Best practice plans have been drawn up by the government for businesses to ensure workers are protected – such as remaining two metres apart, being given face masks or provided with hand sanitiser.
But the Health and Safety Executive (HSE) – which will do spot inspections to make sure firms are keeping people safe – has confirmed any that don't follow the new rules could face legal action.
Chief executive Sarah Albon explained how much power the agency will have to live up to Prime Minister Boris Johnsons demand for any reopening workplace to be "COVID-secure".
At Downing Street's daily coronavirus briefing, she said: "Inspectors can require businesses to do certain things – enforcement notices, requiring them to take particular kinds of action.
More from Covid-19
"In the most extreme circumstances if there is a rRead More – Source