The guidance also means incoming travellers must take a PCR test within two days and isolate until they receive a negative result.

Any person who comes into contact with a confirmed omicron case must self-isolate for 10 days, regardless of their vaccination status. Anyone who breaks the self-isolation law without a “reasonable excuse” faces a fine of £1,000, rising to £10,000 for repeat offenders and serious breaches.

The UK Health Security Agency is carrying out surge PCR testing and enhanced contact tracing at locations where omicron cases have been identified. 

Boris Johnson said the new measures were “proportionate and responsible and will buy us time in the face of this new variant”.

The Confederation of Passenger Transport said on Monday that police, rather than operators, would enforce the new mask laws on public transport.

The organisation, which represents the UK’s bus and coach industry, said: “We expect passengers will work with operators to ensure passengers can travel safely. 

“People can still have the confidence to travel by bus with measures to improve ventilation and tools such as apps to allow customers to see how busy their bus is in place.”

Guidance published by the Department for Education said university students and academics should wear face masks in communal areas like corridors. 

Vice-chancellors were told they “may also want to consider” recommending masks in lecture theatres, teaching rooms and libraries, particularly where social distancing is difficult. 

But Sir Desmond Swayne, a Conservative backbencher, claimed wearing face coverings to provide protection from the virus was “mumbo jumbo”.