One in five hospital patients with coronavirus caught it while on wards, papers by Government scientists have revealed.
The findings come alongside orders to all hospitals to enforce social distancing between staff in order to stop doctors and nurses from "congregating" and fuelling the spread of the virus.
An investigation by The Telegraph reveals that it was not until May 18 – nearly two months after Britain entered lockdown – that health chiefs finally issued guidance on how hospital workers should implement social distancing.
It comes amid growing concern that NHS trusts have become a major source of coronavirus "re-seeding" into communities. Government scientific advisers are worried that, while medics are keeping their distance from patients, their "backstage" behaviour is allowing the virus to circulate.
Government advisers are concerned that high transmission rates within hospitals and care homes are to blame, making it far more difficult to ease the UK's lockdown restrictions.
Papers published by Sage on Friday show that, at the peak of the epidemic, transmission within hospitals was believed to account for up to 22 per cent of hospitalised patients and up to 11 per cent of deaths.
The research, prepared for the Cabinet Office, says these figures do not include staff or those attending outpatient appointments, meaning they are "very likely to be an underestimate".
The failure to stem the crisis has prompted a letter from the NHS England medical director, Professor Stephen Powis, instructing hospitals to "minimise" close contacts between staff, with workers told to "avoid congregating at central work stations" and take staggered staff breaks.
One senior NHS manager said: "The situation is ridiculous. It seems hard to believe, given that the whole country has been talking about social distancing for months, but it is only now that some NHS trusts are really thinking about basic measures like ensuring nurses aren't all hanging out together."
Health chiefs have written to every hospital and GP practice, urging them to make changes in order to reduce levels of transmission of coronavirus within clinical settings.
From next week, all staff, patients and visitors will be required to wear masks in hospitals.
The move was prompted by fears that Britain remains stuck in lockdown because it has been unable to get a grip on outbreaks in hospitals and care homes, which leak out into local communities and increase the risk of a second wave.
While the UK recorded its first case of Covid-19 in late January and entered lockdown on March 23, an investigation by The Telegraph reveals:
The letter from Prof Powis instructs health service hospitals to take "further action to tackle infections acquired in the NHS itself".
The letter, also signed by the NHS chief nurse and operating officer, spells out the importance of "all staff adhering to social distancing (two metres) wherever possible in non-clinical areas".
It follows growing concern that coronavirus is spreading within hospitals because Britain failed to introduce basic measures, such as masks, routine staff testing and strict social distancing, which have helped to contain the virus in other parts of Europe.
The instructions follow the infection of 100 staff at Weston General Hospital, which forced its closure to new patients for two and a half weeks.
Asked about surges of infections in hospitals, Prof Powis said that, as community incidence was falling, the focus would be on "individual outbreaks" where they occur.
He said all hospital workers will wear masks regardless of whether they are on frontline duties in a bid to reduce infection rates.
"When you move out of the ward areas where PPE [personal protective equipment] is worn and into the corridors and spaces where sometimes it can be harder to manage social distancing, it's important that the use of face masks is there to further reduce attack the chance of transmission," he said.
The May 18 updating of the PHE guidance on infection control included advice about how NHS trusts should ensure that staff follow social distancing protocols. This was a full week after the Government had issued advice to other employers considering reopening their workplaces.
The PHE guidance said all staff should adhere to social distancing wherever possible, and ward practice should be reviewed to minimise close contact between groups of staff for prolonged periods and congregation at nurses' stations.
Previous advice had said "social distancing of two metres should be facilitated wherever this is possible", without making any reference to staff.
There are growing fears that the behaviour of health and care workers – who are often working in crowded and stressful situations – is driving up the rate.
Members of the New and Emerging Threats Advisory Group (Nervtag), which advises the Government, are concerned that workers who observe social distancing rules in public behave differently "backstage".
One said: "There is a good bit of anecdotal evidence that the hospital staff are rigorous with infection control on Covid wards when they're doing Covid-related work. But as soon as they come off the wards, you get stories of 10 doctors clustering round an X-ray viewer without any regard to social distancing or masks or anything like that.
"People will sit round and they will have their coffee, and the language that some hospital staff used is that it's 'more like home than home'."
At a webinar late last month, senior officials expressed similar concerns. Dr Chris Moran, the deputy national strategic incident director for Covid-19, told fellow managers: "I've witnessed, and I'm sure you've all witnessed, that actually healthcare workers have not necessarily been the best at managing social distancing.
"We know when directly managing patients social distancing is impossible, that's what PPE is for, to protect both sides of the equation. But I think in the staff-only areas we could do quite a lot better in some of the places that I've visited."
Last week, Sage members warned the Lords science and technology select committee that the situation was putting the rest of the country at risk of a second wave.
Professor John Edmunds said: "These are not completely separated epidemics… they seep back out into the community, probably largely through healthcare workers or care home workers who are picking up the virus themselves, unfortunately, bringing it home, and potentially infecting their family members and others."
The papers released by Sage suggest health officials failed to properly consider the risks of allowing the virus to spread in care homes.
While care home deaths are expected to make up at least half the fatalities caused directly or indirectly by the virus, a paper by PHE on when to abandon the policy of contact tracing said: "Transmission chains in closed settings such as care facilities do not have the same implications as those chains in the community."
The discussions came ahead of the decision to abandon routine testing and tracing of Covid-cases from March 13, leaving patterns of disease unchecked for more than two months.
Research led by University College London Hospitals (UCLH), which was co-authored by Sage member Professor Susan Michie and has yet to be peer-reviewed, has shone a light on how far the infection has spread among staff.
The study found that, of 200 frontline health workers at University College London Hospitals, 43.5 per cent had been infected with Covid-19 by early May.
This was considerably higher than researchers had expected after larger studies of healthcare workers at hospitals in Barcelona and Birmingham found that just 11 and 24 per cent respectively had been infected.
An NHS spokesman said: "Hospitals across the country have long-standing, effective mechanisms for infection prevention and control, and the NHS has regularly communicated the guidance which is set by PHE, as well as supporting trusts with data and a checklist to share learning and help limit the spread of infection.
"The Government has always been clear that social distancing is recommended for everyone."
PHE said the four UK chief medical officers, alongside PHE and the NHS, jointly issued infection prevention control guidance for health and social care workers on April 24, which reinforced existing guidance for the UK that "social distancing of two metres should be facilitated wherever this is possible".
The guidance did not suggest how this should be achieved.
Duncan Selbie, the PHE chief executive, said the Sage report on transmission of spread in care homes "highlighted that spread in the community operates differently to spread in a care home – it does not say that transmission could not occur from a care home into the community."