In the South East, where the variant was most dominant, it accounted for 80 per cent of positive test results in care home staff and residents processed in the second week of December and analysed by researchers from University College London (UCL).
The researchers said the timing of infections suggested the new variant may have been passed from staff to residents, with positive cases among older people occurring later.
They studied 4,442 positive polymerase chain reaction (PCR) tests undertaken by care home residents and staff and processed between 23 November and 13 December.
These were processed at the Lighthouse laboratory in Milton Keynes, one of the UK’s biggest coronavirus testing labs.
The team behind the study were able to detect which infections were from the UK variant by identifying samples that tested positive on two gene targets but not on a third, the S gene, which is modified in the B.1.1.7. variant.
The variant accounted for 12 per cent of positive care home tests in the week starting 23 November, rising to 60 per cent in the week beginning 7 December.
Over the same period in the southeast, positive tests due to the variant rose from 55 per cent to 80 per cent, and in London increased from 20 per cent to 66 per cent.
The majority of samples were from care homes in the southeast, West Midlands, London, or the East Midlands, with the results unlikely to be representative of all care homes across England.
Senior author Dr Laura Shallcross, from the UCL Institute of Health Informatics, said: “Our findings suggest the UK variant spread just as quickly in care homes as it did in the general population.
“This shows the importance of public health measures to reduce transmission in the country as a whole.”
Lead author Dr Maria Krutikov, also from the UCL Institute of Health Informatics, added: “Our results are consistent with national trends, suggesting the UK variant was present in care homes from early on, although our sample did not fully represent all care homes in England.
“As we carried out this work in December, we were able to inform public health decisions at the time.
“To see how viruses like Covid-19 are changing and to respond quickly and appropriately, it is really important we have an advanced surveillance system, with gene sequencing that can identify new variants as early as possible.”
The research was conducted as part of the Vivaldi study looking at Covid-19 infections in care homes, and received funding from the Department of Health and Social Care.
It is published as a letter in The New England Journal of Medicine.
A spokesperson for the Department of Health and Social Care said: “Protecting residents in care homes is a key priority and we are doing everything we can to support the sector, including offering free PPE, regular testing and providing over £1.1bn for infection control measures.
“In the face of the new variant, we acted to protect those most at risk in care homes, with an additional £269m to fund tests and increase staffing. In addition, we have prioritised residents for a vaccine in line with advice from the independent JCVI.
“This study helped inform our response to the new variant and the national restrictions and vaccination programme are helping to reduce transmission. It is vital we all continue to follow the latest guidance to reduce infections, stay safe and save lives.”
Additional reporting by PA