Matt Hancock, the Health Secretary, took a swipe at the Mayor of London on Tuesday night for failing to run a full Tube train service amid fears that packed carriages are putting key health workers in danger.
Mr Hancock's intervention, live on television, came at the end of a dramatic day in which Boris Johnson and the Mayor, Sadiq Khan, blamed each other for overcrowded trains across the capital.
Mr Khan said he had pleaded with the Prime Minister to shut down construction sites over fears that builders are spreading coronavirus at work and on public transport but was overruled by him at a Cobra emergency meeting on Monday evening.
On Tuesday, he accused Mr Johnson of "moving too slowly" to curtail the spread of coronavirus.
The new crackdown announced by the Prime Minister on Monday appeared to be undermined by his insistence that builders, despite not being classified as key workers, could carry on working regardless.
Under the Downing Street rules issued on Monday night, people can leave the house if they are "travelling to and from work, but only where this absolutely cannot be done from home".
That policy seems to have given the green light for builders, cleaners and others to continue work as normal. In contrast, construction sites have been shut down in Scotland on the orders of Nicola Sturgeon, the First Minister. Mr Khan's plea at the Cobra meeting was also backed by Welsh leader Mark Drakeford and Northern Ireland's Arlene Foster.
On the first day of the most stringent lockdown since the war, health workers were forced to go to work on crowded trains and buses caused by a sharp reduction in public transport.
One hospital health worker told The Telegraph she was appalled that she was being put in danger and pleaded with the Prime Minister and the Mayor to institute a ban on non-essential workers using the Tube. Transport for London (TfL), which runs the London Underground, said it did not have the capability or powers to enforce such a ban.
Mr Hancock waded into the growing row between Downing Street and City Hall by insisting that Mr Khan ensured the Tube ran to a normal timetable that would give workers the chance to keep a safe distance.
TfL cut services back last week, insisting the Tube should only be used by critical workers such as nurses and doctors, and made further reductions on Tuesday after almost a third of Underground staff went off sick or into self-isolation.
In the daily televised press conference, Mr Hancock – in a direct swipe at Mr Khan, who is in overall charge of public transport in the capital – said: "When it comes to the Tube, the first and the best answer is that Transport for London should have the Tube running in full so that people travelling are spaced out and can be further apart, obeying the two-metre rule wherever possible.
"And there is no good reason in the information that I've seen that the current levels of Tube provision should be as low as they are. We should have more Tube trains running."
The Prime Minister has also told Mr Khan to put on more trains, while Grant Shapps, the Transport Secretary, said he had requested from the Mayor measures "to ensure the safety of those who need to use it [the Underground]".
In a further sign of tension, Kit Malthouse, the policing minister and a former deputy Mayor when Mr Johnson ran City Hall, was put in charge of a new committee within the Home Office to tackle the growing crisis in London, the epicentre of the UK coronavirus outbreak.
Mr Shapps said one option was to bring drivers and signallers out of retirement "to assist in this great national effort to beat coronavirus".
Mr Khan blamed Downing Street on Tuesday for not shutting down construction sites in London and failing to offer a compensation deal to low-paid, self-employed workers that would keep them at home.
He said in a radio interview that he had called, at the Cobra meeting, for building work to be halted on non-essential jobs but explained: "I was overruled by the Prime Minister, who doesn’t believe that construction workers should be at home.
"The Prime Minister believes construction workers should be going to work and they can do it safely. I’ll tell you this, I’ve worked on a building site, I’ve been a labourer, but also I visit building sites.The idea that construction workers can stay two metres apart during the course of a busy day, but also the idea we can’t put on hold certain construction work in light of this public health crisis, I find astonishing.
"The government… have to realise that people are dying. The Prime Minister has been moving slower than I would have wanted. I still don't think last night's message was as clear as it could be, and this conversation is evidence of that."
The simmering row between Downing Street and the Mayor's office blows apart any attempt at a consensus on how to crack down on the spread of Covid-19. London has about a third of all cases in the UK, while ministers fear the capital has become a "city of superspreaders".
Nicola Smith, who carries out ultrasound scans at a central London hospital, posted a picture of her journey to work online, showing Tube passengers squeezed together.
"This is my Tube this morning. I live in zone 4 and work in a zone 1 hospital. I love my job, but now I'm risking my health just on the journey in?!" she wrote on Twitter.
She called on the Mayor and Prime Minister to "start policing" who was allowed to use public transport, telling The Telegraph it was important for the public to "see how busy the Tubes still are".
The site manager of a construction project in central London contacted The Telegraph on Tuesday to say it was "so immoral" that his team were being forced to continue work.
"There are 60-year-old men crying because of the impact it will have on their family," he said. "No one wants to be here, but 80 per cent are self-employed and our business is standing by the mantra that we are key workers."
The site manager said there were 120 builders working on site and just one cleaner. "We have a canteen and drying room completely full of guys,” he said, explaining that keeping a distance of two metres while working was impossible to achieve. He added: "We had more than 60 guys in the canteen I'd say. At one time. Despite staggered breaks."
The Prime Minister's spokesman confirmed building work was allowed to continue, saying: "In terms of construction, it should continue where it can happen in a way that allows for Public Health England and industry guidance to be followed.
"We urge employers to use their common sense when managing live projects. Employers should ensure safe social distancing on site. That applies to construction in general – it should continue where workers can keep two metres away from each other at all times."
The spokesman acknowledged that "the PM raised with the Mayor the issue of reduced services on the Tube", but added: "This is a matter for TfL. The Transport Secretary has also spoken to the Mayor where they looked at ways of making sure transport timetabling is in place to ensure the safety of those who need to use it."
Manuel Cortes, the general secretary of TSSA, the travel and transport union, criticised ministers for insisting London Underground should run a normal schedule.
He said: "The idea that the Tube can run with a normal service level is utter nonsense. Thirty per cent of Underground staff are either off sick as a result of Covid-19, or in self-isolation."