PRESSURE is mounting on Boris Johnson to take the "difficult decision" and ban non-essential building work as a row broke out over construction workers contributing to overcrowded trains.

The focus of the row has been the London Underground, where, just hours after the Prime Minister’s plea for people to stay at home or, if outside, observe the self-distancing rules, early day commuters crammed onto Tube carriages.

British Transport Police pointed out it had an important role to help ensure only those people making “essential journeys for work” were using underground and overground rail networks and stressed its officers would be “on hand to support rail operators if people are clearly disregarding the advice to help slow the spread of the virus and keep people safe".

Last night, the office of London Mayor Sadiq Khan called on the UK Government to act urgently to get more people to stay at home in the light of construction workers reporting to building sites in the UK capital and photographs on social media showing crowded trains.

The response came after Matt Hancock, the UK Health Secretary, at the daily Downing St press conference, argued that the Tube should be running “preferably in full,” so that people could be spaced out and can be further apart.

He also repeated the earlier Downing St line that construction workers could continue to go to work as long as they were able to remain two metres apart at all times.

But a spokeswoman for the Mayor said the Government needed to provide "proper financial support" to freelancers and the self-employed.

She explained: "The Government must act urgently to get more people staying at home rather than going to work unnecessarily; that means taking the difficult decisions they are refusing to take to ban non-essential construction work and provide proper financial support to freelancers, the self-employed and those on zero-hours contracts to stay at home."

Earlier, asked if construction workers in London should be among Tube travellers, Mr Khan said: “No and I made that point quite forcibly at yesterday's Cobra; I made that point quite clearly to the Prime Minister. According to the Government's advice, the answer is yes."

Calling for people to “stop all non-essential travel," he demanded employers allow their staff to work from home "unless it's absolutely necessary", adding: "Ignoring these rules means more lives lost."

In Edinburgh, Nicola Sturgeon also argued that all non-essential building work should cease. The First Minister explained it was clear construction sites should "be closed unless the building that is being worked on is essential, such as a hospital".

Transport for London[TfL] - which is chaired by Mr Khan - has suspended the Circle line and Waterloo & City line, and reduced frequencies on other parts of the Tube network. Bus services have also been cut.

The Mayor said "growing numbers" of TfL staff were off sick or self-isolating, which meant “we cannot run more services than we currently are".

Calling for urgent UK Government help for the self-employed to encourage them to stay at home, he said: "Many of those still travelling to work today are on zero-hour contracts, work in the gig economy or are freelancers.

"A proper package of support for these workers would alleviate this situation and help public transport and I've raised this with the Government," added Mr Khan.

Meanwhile, the union Unite said construction workers were facing the choice of risking their health or losing their job.

Gail Cartmail, its Assistant General Secretary, said: "There is an immediate public health emergency on construction sites due to a lack of social distancing.

"By construction workers being compelled to work unprotected and travel, the lack of Government safety co-ordination, is risking their health, the health of their families and the health of the general public."

Yet No 10 defended its approach, saying construction work should continue if it could be done following Public Health England and industry guidance.

The Prime Minister's spokesman pointed out the Construction Leadership Council had issued guidance to the industry.

“It should continue where it can happen in a way that follows Public Health England and industry guidance. We urge employers to use their common sense when managing live projects and ensuring that employees can follow the Government guidance and practice safe social distancing on site," he added.

Meanwhile, a crane operator at a London construction site with 400 to 500 workers said: "Everyone on site at the minute feels angry and unprotected."

The man, who asked not to be named, said if he opted not to go in he "wouldn't get paid or even could lose my job".

He added: "[It's] scary really. I've got a family and kids at home. I'm in London around all these people, bringing it all back into an isolated home."

Electrician Dan Dobson said the Government must provide support to self-employed workers, including around one million in the construction industry and then shut down building sites.

He added: "They have no incentive to stay at home, they have bills to pay…None of them wants to go to work, everyone is worried about taking it home to their families.

“But they still have bills to pay, they still have rent to pay, they still have to buy food. Construction sites will stay open until the Government issues an order to close.

"But the Government cannot issue the order to close until it offers support to the one million-plus construction workers - it has to go hand in hand," he added.

While some companies said they were closing sites down and suspending work, others said they would remain open with "strict precautions" in place.

TfL announced on Tuesday work on its Crossrail sites was being temporarily suspended, but that essential maintenance of the transport network will continue.

HS2 said it would be reviewing the majority of works on its construction sites in line with the Government's advice on dealing with Covid-19.

House-builder Taylor Wimpey also said it had closed its construction sites, show homes and sale sites.

But rival Redrow said its sites remained open with "strict precautions in place including enhanced levels of cleaning, additional hygiene facilities and social distancing".