London's pubs may be ordered to close but city will stay open

"There are no plans to close down the transport network in London and there's zero prospect of any restrictions being placed on people traveling in or out of London," Johnson's spokesman James Slack told reporters. It is "not true" that only one person will be allowed out of their home at any one time, he added.

The prime minister is threatening to tighten restrictions in the capital, which is at the center of the outbreak, in order to enforce so-called "social distancing" policies. Measures under consideration include ordering bars and restaurants to close, and taking a tougher approach to ensure more people work from home where they can, according to a person familiar with the matter.

Limits are also being placed on London's transport network with dozens of stations closing, and Johnson was scheduled to meet London Mayor Sadiq Khan in Downing Street on Thursday to discuss the next steps.

Earlier, Education Secretary Gavin Williamson used the word "lockdown" when he said no decision had yet been taken on action in the capital.

Johnson, who has faced criticism for moving too slowly against the disease, is now racing to slow the spread of the coronavirus, which has already killed 104 people in Britain. The virus is spreading fastest in London, according to the government.

When asked about the prospect of a lockdown, which was widely reported in the U.K. media on Thursday morning, Slack reiterated the advice on keeping away from other people and not attending gatherings.

"The prime minister has been clear that what we want people to do is to follow the very good advice, which is particularly pertinent in London, that people should avoid unnecessary social contact to limit the spread of this disease and save lives," he said.

As many as 20,000 military personnel are on standby to be deployed to support civil authorities as part of a new "Covid Support Force." They will include people trained to drive oxygen tankers to hospitals, as well as scientists working on combating the deadly disease.

The measures are part of contingency plans to respond to requests from governments departments or civil authorities. The troops will potentially take over guarding sensitive sites to free up police officers for frontline law enforcement duties. Soldiers will also be trained in the logistics of driving oxygen delivery tankers to hospitals.

Transport for London announced the closing of 40 underground stations as it encouraged people only to make essential journeys and to leave the network of subways, buses and trains free for the use of workers vital to the effort against the pandemic.

"People should not be traveling by any means unless they really, really have to," Mayor Sadiq Khan said in a statement. "London will get through these extraordinarily challenging times, and ensuring the capital's critical workers can move around the city will be crucial."

Johnson and Khan will discuss the closing of underground and mainline stations, restricting bus services and possibly closing some underground lines entirely, a person familiar with the matter said.

Transport Secretary Grant Shapps said the government is working with airlines and airports to develop a package to help them weather the impact of the virus.

"Coronavirus is having a crippling impact on the aviation industry and we cannot allow it to force world-leading, well-run, profitable firms out of business," he said.

Johnson's government is also expected to publish emergency legislation on Thursday that will give it the powers to close meeting places, ban gatherings and detain people who are a danger to public health as it seeks to halt the spread of the virus.

Failure to slow the contagion may require tighter controls on the movement of people, Johnson said on Wednesday. The government says it already has the power to keep individuals in isolation for their own safety.

"We live in a land of liberty as you know, and it's one of the great features of our lives that we don't tend to impose those sorts of restrictions on people," Johnson told reporters in London. "But I have to tell you, we will rule nothing out."