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Sadiq Khan has challenged official government policy by ordering compulsory mask wearing on London's buses, tubes and trams.
Ministers are set to drop mandatory face coverings when most Covid lockdown restrictions are lifted on Monday July 19th.
But the mayor has decided to make face coverings a ‘condition of carriage’ on all Transport for London services, including the Overground and Docklands Light Railway.
TfL is also looking at ways to make taxi and minicab drivers and passengers obey the mask order.
Khan said that 4,000 fixed penalty notices have been issued over the last 11 months.
When asked if other regional leaders may follow London's lead, he added:
"We have been in contact with transport providers and leaders across the country. It is for each area, each transport provider to decide what they want to do. I am quite clear from the evidence I have seen from Sage and the WHO that you wearing a face mask is probably the most unselfish thing you could do."
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The move means different rules for the TfL network and mainline trains. From Monday commuters can choose to abandon masks on some trains but will be forced to cover up if they switch to TfL services.
City Hall said enforcement officers would patrol the TfL network. Passengers could be turned away from bus stops or stations or ordered to leave trains and buses.
More than 200,000 people were made to put on a mask before travelling in the last 12 months.
"I’ve repeatedly made clear that the simplest and safest option would have been for the Government to retain the national requirement for face coverings on public transport," said Sadiq Khan.
"I’m not prepared to stand by and put Londoners, and our city’s recovery, at risk.
"This is why, after careful consideration, I have decided to ask TfL to retain the requirement for passengers to wear a face covering on all TfL services when the national regulations change." Mr Khan added.
Prime Minister Boris Johnson outlined official government advice on Monday calling on people in England to consider wearing a face covering in confined or crowded spaces.
Mr Khan said his instruction would give passengers confidence and protect transport workers and vulnerable travellers.
The Transport Secretary supported Sadiq Khan's decision was “very much in line” with what ministers wanted to happen, despite Boris Johnson’s decision to lift legal restrictions in England next Monday.
Grant Shapps told ITV News: "I don't buy the idea that this is hugely complicated and we are moving into Step 4 (of easing restrictions). We are releasing people from these legally required mandates. But that does not mean we don't want people to be sensible and we invite transport operators do do what is right in their circumstances."
However the RMT union warned railway workers face the threat of violence because of the “botched and confused” approach to face coverings on public transport.
General Secretary Mick Lynch said: "Whilst we welcome the approach from the London Mayor this morning, which is consistent with the policies currently adopted in Scotland, Wales and on Eurostar, we now have the ludicrous position where a passenger travelling through London will have different rules on the Tube and the mainline services.
"There will also be a change of policy on trains at the Welsh and Scottish borders, which is a total nonsense and will leave staff right at the sharp end and dangerously exposed when it comes to enforcement.
"As a result of this chaotic approach we now have a situation where the London measures are not enforceable by law, which means RMT members will be thrown into a hostile and confrontational situation from next Monday at heightened risk of abuse and assault.
"That is wholly down to the confused, inconsistent and botched messaging from the Government."
Public health expert Linda Bauld said she backed the decision on face masks in London because of the risks.
Three-quarters of Britons are likely to continue wearing face coverings in shops and while using public transport even when they are no longer compulsory, polling has found.
According to a survey by Ipsos Mori, 76% think it is likely that they will put a face covering on when going into shops after restrictions have been eased, while 74% said the same when it came to taking public transport.
A majority feel they are likely to wear them on planes (64%), in theatres and cinemas (60%), in their place of work (59%) and in pubs and restaurants (55%) even when no longer legally compelled to do so.