Shops will be able to refuse entry to people who do not wear face masks, and can call the police.
On the other side of the English Channel, amid signs of a slight virus resurgence in France, President Emmanuel Macron said he also wants to require masks inside all indoor public spaces by August 1.
"Certainly, face coverings would be one of those, but it would be at the end of it quite a long list".
Hancock stressed that wearing a face covering complements the other measures aimed at keeping a lid on the pandemic, such as washing hands and abiding by social distancing rules.
During a visit to the London Ambulance Service on Monday, Boris conspicuously staged a photo op wearing a mask and also offered the clearest signal that the government was looking at "tools of enforcement".
"There is growing evidence that face coverings can stop the spread of coronavirus and I would urge residents to play their part and wear a face covering when out and about".
Dr Zubaida Haque, a member of the Independent Sage group, set up to scrutinise the government's coronavirus policy, said the £100 face mask fines were "absolutely wrong in the absence of a public information campaign". It will be reduced to 50 pounds if paid within 14 days.
A RMT spokesperson said: "The Rail Industry Coronavirus Forum has agreed that Rail staff can not "police" the use of face-coverings on the Rail Network".
Face coverings are not the same as face masks.
Some people have criticised the 11-day gap between announcing the rules and enforcing them, but realistically you have to give people time to find and buy masks.
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Boris Johnson confirmed face masks will be mandatory in shops and supermarkets. In addition to having ear loops, the mask has an elastic strap along the back, which reduces the pressure on the ear straps, making it more comfortable to wear for extended periods.
As Lillis asked, how are shops supposed to check under-age ID when a face is covered?!
However, children under the age of 11 and people with particular disabilities will be exempt from the rules.
But she also warned: "Wearing a face covering will not prevent you from catching the virus, but it could prevent you from unknowingly spreading it to other people".
Can I wear a face shield or visor instead?
Michael Gove said over the weekend that the public could be trusted to use their "common sense" in choosing whether to cover their nose and mouth, but now it seems only the threat of a fine will do.
"However, it is vital that we remember not everyone is able to wear a face covering".
In Wales, the advice is to wear a face covering that consists of three layers.
It emphasises that a mask alone can not protect you from Covid-19, and that it must be combined with social distancing of at least a metre and regular hand washing. Click the link in the orange box below for details.