More than 400 care home residents were dying every day with COVID-19 at the height of the pandemic, analysis has shown.
In just one week in April, there were more than 3,000 coronavirus-related care home deaths in the UK, according to findings by the PA news agency.
Deaths rose by 500% between the 1 and 17 April of the month, when almost 500 residents died with COVID-19 across the UK.
On 17 April, 490 residents died - making it the deadliest day for care home deaths.
It comes at a time when testing for all staff and residents, regardless of any symptoms, was not in place, and providers had not yet been told to restrict the movements of staff between venues.
Carl Heneghan, professor of evidence-based medicine at the University of Oxford, believes that staff movement could have been of "huge" significance in allowing the infection to spread.
Not only that, but from mid-March, thousands of hospital patients were placed in care homes, and there were well-documented struggles in accessing PPE (personal protective equipment).
By analysing data from the UK's various statistics agencies, PA also found:
By 26 April, when Northern Ireland became the first UK nation to tell providers staff movements should be limited, there had been 9,074 deaths in care homes - 51% of the total number up to 31 July.
By 28 April, after England became the first UK nation to announce it would be testing all care home staff, regardless of symptoms, 9,776 residents had died across the UK.
It took 43 days for the number of people to die in care homes to reach 5,000 - but only 12 days for that number to double
In July, England's chief medical officer Chris Whitty said that "major risks" in care settings had not been considered early on, such as staff working in multiple homes and those not being paid sick leave.
It took two months for Public Health England to change its March guidance that care homes should share staff to advice that carers' movements should be restricted.
Professor Heneghan says this was due to a lack of experience in the government's advisory team.
"The government should have advised that agency staff, if used, should be employed in a single care home and not travel between multiple care homes. That advice should have been given in the middle of March," he said.
SAGE, the Scientific Advisory Group for Emergencies, had previously discussed the potential risks of staff movement at a meeting in April.
Care England chief executive Professor Martin Green said that advice had been "slow to come to fruition", particularly with regards to visits.
He said: "Many care homes locked down before national guidance came into force.
"Unfortunately patients were discharged from hospital without testing and this, compacted with insufficient PPE, created huge challenges for care homes.
"Routine testing is absolutely essential in order to establish confidence in the system for residents, staff, relatives and beyond."
A Department for Health and Social Care spokesman said: "We have been doing everything we can to ensure care home residents and staff are protected, including testing all residents and staff, provided 200 million items of PPE, ring-fenced £600m to prevent infections in care homes and made a further £3.7bn available to councils to address pressures caused by the pandemic - including in adult social care.
"As a result of actions taken, almost 60% of England's care homes have had no outbreak at all and the proportion of coronavirus deaths in care homes is lower in England than many other European countries."
Analysis: 'The data will raise more questions for the NHS'
By Isla Glaister, data editor
This latest analysis of the care home data will once again raise questions about NHS England's hospital discharge policy.
Trusts did not have to test patients before discharging them into a care home until 15 April.
As the data shows, deaths related to COVID-19 were already peaking at this time, increasing five-fold in the previous two weeks.
Neither the government, nor the NHS, have yet confirmed how many patients were sent to residential homes without passing a test.
But, Sky News recently obtained figures suggesting at least 4,000 people were released without taking a test, and more than 600 after testing positive for coronavirus.
Excess deaths were higher in England than other UK nations, 80% above the five-year average, according to a study by researchers at Stirling University - but the virus has devastated the entire UK care home sector.
The latest Office for National Statistics report shows around one in 25 of the 500,598 care residents have died with confirmed or suspected coronavirus since March.
This is 40% of all deaths registered as related to COVID-19 and, given gaps in testing and other data, it is likely to underestimate the true impact."