London's mayor has written to all major supermarkets calling on them to stock face coverings to help protect people using the capital's public transport.
In his letter, Sadiq Khan asked that masks should be reusable, non-medical and at a price which was "accessible to all".
The government has advised coverings should be worn in an enclosed space when social distancing is not possible.
Mr Khan said face masks would be "essential" as the lockdown was lifted.
The letter urges supermarkets and shops "to stock supplies of reusable, non-medical face coverings in all of your London stores".
"Whilst we at City Hall are encouraging the use of homemade face coverings, I recognise that there will be many Londoners who will need or want to purchase a ready-made product instead," Mr Khan wrote.
He added those coverings going on sale should not be medical-grade masks in order to protect supplies for front-line workers.
"Given that I am asking the public to use face covering when on public transport, there will be a huge demand for them in the coming weeks," he said.
The British Independent Retailers Association (BIRA) has also called on its members "to play their part in keeping customers safe" by stocking face coverings.
Chief executive Andrew Goodacre said: "As they tend to sell personal protection equipment (PPE), we want to ensure local hardware stores ramp up supply to keep the public safe, to help people avoid needing to travel to the larger, more populated, out of town DIY stores to buy masks."