The government has written to care home providers in England to warn them of a rise in new coronavirus infections within the sector.
A letter from the Department of Health urged care bosses to take "necessary action to prevent and limit outbreaks".
Cases were mainly among staff but risked spreading to residents, it said.
It comes as a further 3,330 positive cases were recorded in the UK - the third consecutive day in which cases have been over 3,000.
It brings the total number of confirmed cases to 368,504.
Five more deaths of people who had a positive test within the previous 28 days were also reported. This brings the UK total to 41,628.
Justice Secretary Robert Buckland told the BBC the key to preventing mass outbreaks was to ensure availability of testing for care homes.
Friday's letter from the Department of Health and Social Care said testing data had revealed an increase in the number of positive results in care homes and called on the care sector to work with the government.
"You will know already that we are experiencing a rise in confirmed Covid-19 cases across the UK population," wrote Stuart Miller, director of adult social care delivery.
"I need to alert you to the first signs this rise is being reflected in care homes too.
"I am writing at the earliest opportunity, so we can work in partnership to prevent further spread of the disease. The rapid flow of data and information, to and from care providers, is vital to this effort."
Mr Miller said the infections had been detected chiefly among staff but had been transmitted to residents in some cases. He went on to stress "the importance of regular testing and consistent use of PPE".
'Looking over our shoulder'
Speaking to the BBC, social care provider Mike Padgham - who runs four care homes in North Yorkshire - said the letter "doesn't tell us anything we didn't know already".
He said there were currently no cases of coronavirus in any of his care homes, but called for clarity around visiting, as the R number - the reproduction rate of Covid-19 - has recently increased.
"At present, things are fairly calm, but we are looking over our shoulder at what is coming next," he said.
Mr Padgham, regional chairman of the Independent Care Group, said one of his homes had reopened to visitors only last week for the first time since March, but since receiving the letter, he was now wondering if visits were a good idea.
He said his "natural instinct was to allow visits" - in keeping with families' wishes - but added he "would like more guidance".
"You're damned if you do, damned if you don't…" he added.
Earlier, Justice Secretary Robert Buckland commented on an article in the Sunday Times, citing a leaked report, that positive cases of coronavirus had been identified in 43 care homes across England.
He told BBC One's Andrew Marr programme the key to preventing mass outbreaks was to ensure "availability of testing" for care homes.
"With that combination of testing and the rules that are applied by the care homes we can do justice to the elderly people in our care," Mr Buckland said.
It comes after the UK recorded more than 3,000 new cases of coronavirus for the second consecutive day on Saturday.
Peter Openshaw, professor of experimental medicine at Imperial College London, warned the UK "must act now" to keep coronavirus transmission under control.
He told Sky's Sophie Ridge: "It's starting to appear in people more vulnerable and that, inevitably, is going to be followed by hospital admissions and deaths so we need to act quickly."
A DHSC spokeswoman said: "Throughout our coronavirus response we have been doing everything we can to ensure all staff and residents in care homes are protected.
"We are testing all residents and staff, have provided 200 million items of PPE and ring-fenced £600m to prevent infections in care homes, with a further £3.7bn available to councils to address pressures caused by the pandemic, including in adult social care.
"There is a high demand for tests and our laboratories continue to turn test results around as quickly as possible and we plan to rapidly expand it in the coming weeks as well as bringing in new technology to process tests faster."