Unite said their members had been doing "a heroic job in getting NHS and care workers to their places of work" and described the loss as a "tragedy".

Regional secretary Peter Kavanagh said the union would be assisting families "in every possible way".

Transport for London (TfL) said it was "extremely saddened" by the deaths, while mayor of London Sadiq Khan said he was "absolutely devastated".

It is understood three were drivers and two were controllers.

'Heroic effort'

Unite said it had contacted Mr Khan, who shared its view "that bus drivers must be fully protected".

"We are absolutely committed to doing everything in our power to make the driving of buses safe during this unprecedented crisis," said Mr Kavanagh.

Current safety measures include deep-cleaning vehicles, erecting screens around the driver, providing hand sanitiser and blocking off the seats closest to the driver.

Mr Khan said he will continue to make enhancements across public transport in London to ensure there are even higher levels of protection.

"My thoughts are with their friends and families at this awful time," he said.

"I have been clear that our incredible public transport staff, on the buses, tubes, trams and trains, are critical workers, making a heroic effort to allow our NHS staff to save more lives.

"But we all need to play our part too and that means fewer Londoners using the public transport network. Please follow the rules. Stay at home and do not use public transport unless it is absolutely unavoidable."

Gareth Powell, TfL's managing director for surface transport, said they had been offering the bus companies for whom the drivers worked "every support possible".

"The safety of our staff and customers is our absolute priority and we have been working closely with the bus companies, the mayor and Unite to implement a range of changes and improvements to keep the bus network and garages safe for those operating and using it, in accordance with Public Health England advice."