“But I have to tell you we will rule nothing out and we will certainly wish to consider bringing forward further and faster measures where that is necessary to suppress the peak of the epidemic to protect our NHS, to minimise casualties, and to minimise suffering.”

He added: “Absolutely we do not rule it out, because it would be quite wrong to do so. We do not rule out taking further and faster measures in due course.”

The Telegraph understands that London is likely to go into a lockdown as early as the weekend under the most dramatic measures yet to curb the spread of Covid-19. London has more cases than anywhere else in the country (see map below).

UK coronavirus cases tracker

Supermarkets, corner shops selling food  and pharmacies would remain open under plans to be discussed at the latest Cobra meeting.

Officials believe that London, with its population of nine million people, has become a “city of superspreaders”, prompting the Government to consider its effective closure.

Official figures from Public Health England show 929 cases recorded in London, more than a third of the UK confirmed total of 2,626. In reality tens of thousands of people in London will already have the disease.

A well-placed Whitehall source said an announcement would be made “by the weekend” and is likely to be put in place in time for Saturday night. Downing Street declined to rule out the introduction of measures by Friday.

The closure of bars and restaurants will have a devastating effect on the economy and threatens to put hundreds of thousands of people out of work. It is estimated that one in seven people in London work in the tourism, hospitality and leisure sector.

The Whitehall source said public transport would be limited, although it is not clear how the measure will be enforced. It is understood that the wider population will still be able to travel across London in their cars and on bikes.

An internal Transport for London (TfL) memo said: “We must provide transport services for essential workers and keep transport going to critical locations such as hospitals.”

There is alarm in Whitehall that Mr Johnson’s call for the public to avoid pubs and restaurants and to work at home is being ignored in large numbers.

New figures from TfL show a 19 per cent decline in passenger numbers on the Tube – still equivalent to four million journeys a day – and a 10 per cent reduction on the buses to about 5.5 million daily trips. This week, commuters on the Tube were still complaining of overcrowding after it emerged that TfL had begun to reduce the numbers of trains, “matching service levels to the actual demand for travel”.

Anecdotal reports suggest pubs remain packed after work, although plenty of others reported that premises were empty. On Twitter, Francesco Ragni, an Italian writer who lives in London, posted pictures of his packed local pub, adding: “Restaurants and pubs closed? Well, not exactly. Plenty of people at my local pub tonight, enjoying a pint and some food and company. Boris coronavirus advice clearly ignored by many.”

Civil servants have opened talks about the restrictions with staff in the London Mayor’s office and other public bodies about coordinating the action. The timescale of the changes is still fluid to give Boris Johnson the option of ordering it this weekend or delaying by a few weeks.

Sadiq Khan, London’s mayor, has complained that he is not being kept informed about the UK government’s plans and his aides are unsure whether the lockdown will take place, or what form it might take.

A source close to the Mayor said: “We expect a shutdown of London but nobody in central Government is saying when that will be or what form it will take. The Prime Minister is saying we need to go faster and further and there is a spike of cases in London which is running ahead of the country but what that actually means the mayor’s office doesn’t yet know because Downing Street hasn’t said.”

Nicola Sturgeon, Scotland’s First Minister, said she believed the UK Government was about to announce “more stringent measures” for London. Mr Johnson has already admitted that London has become the country’s worst affected region.

Asked how far the UK was from a “complete lockdown”, Ms Sturgeon said the capital could see tougher measures first because it was further ahead on the “curve” of the outbreak. Ms Sturgeon receives briefings on UK-wide coronavirus measures.

Mr Johnson’s allies said no new measures were planned for the capital on Wednesday or Thursday, but Government insiders said the lockdown could be announced on Friday. “People are looking at it, but it’s too early to say when or whether we will do this,” said another government official.

Asked if people could be barred from leaving home without paperwork to show they have a legitimate reason, Mr Johnson’s spokesman said: “We’ve set out the steps which we think are necessary at this point in time.

“But we will be guided by scientific and medical advice to ensure we take the right steps to protect the public.”