Fining passengers who refuse to wear a face coverings on the Tube will "greatly help with compliance", the Mayor of London Sadiq Khan has said.
Masks will be compulsory in shops and transport from Tuesday in response to new Omicron Covid-19 variant cases.
Masks remained a condition of carriage after Covid restrictions eased in July although compliance waned after the rule stopped being legally enforceable.
London TravelWatch said mandatory mask wearing would be "reassuring for many".
Two further cases of the newly identified variant were discovered in London on Tuesday, bringing the UK total to 11.
London was the first English city to insist on face coverings after Covid restrictions eased.
However, a lack of legal powers to enforce the rule meant that Transport for London's (TfL) team of around 500 enforcement officers was left to ensure compliance across the capital without the support of the British Transport Police.
Just 126 passengers have been asked to leave TfL services for refusing to wear a mask since July, despite up to five million passengers using the Tube each day.
Mick Lynch, general secretary of the Rail, Maritime and Transport (RMT) union said 'We support the wearing of masks but there are major issues about enforcement and its our members left in the front line with angry passengers who refuse to comply.
The rules for masks will apply in shops and on public transport but, unlike previously, won't apply in pubs and restaurants in England when not seated.
Sadiq Khan said "we have been operating with one hand tied behind our back" without the ability to fine those that flout mask wearing rules so "wholeheartedly welcomes" the government's decision to once again make masks compulsory.
He said: "Evidence shows that face coverings help stop the virus spreading, and ensuring that this is enforced will greatly help with compliance.
"We know that by wearing face coverings in enclosed or crowded spaces, washing our hands regularly, ensuring indoor spaces are properly ventilated, and maintaining a social distance wherever possible, viruses find it much harder to spread."