The Indian coronavirus variant is now the UK's dominant strain, Public Health England (PHE) has confirmed.
It comes as cases involving the variant, which was renamed the "Delta" variant by the World Health Organization, rose 79% in the space of a week.
As of Wednesday, PHE said there had been 12,431 confirmed cases of the B.1.617.2 variant: up 5,472 from 6,959 a week ago.
As a result, it has overtaken the Kent variant – which caused the UK's winter crisis of huge case numbers, hospitalisations and deaths – as the dominant strain.
Dr Jenny Harries, chief executive of the UK Health Security Agency, said: "With this variant now dominant across the UK, it remains vital that we all continue to exercise as much caution as possible.
"The way to tackle variants is to tackle the transmission of COVID-19 as a whole. Work from home where you can, and practice ‘hands, face, space, fresh air’ at all times.
"If you are eligible and have not already done so, please come forward to be vaccinated and make sure you get your second jab. It will save lives."
Meanwhile, PHE said there is "early evidence" the Indian variant infections may have an "increased risk of hospitalisation" compared to the Kent variant.
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It said 278 people with the Indian variant had attended A&E in the past week, resulting in 94 people being admitted to hospital overnight. Last week, 201 people attended A&E, with 43 admissions. The majority of these people had not been vaccinated.
The latest data, and Dr Harries' words of caution, will raise further doubts over the government's plans to end England's lockdown on 21 June.
However, there was a more pessimistic tone from health secretary Matt Hancock on Thursday. He said ahead of a G7 health ministers’ meeting: “It’s too early to say what the decision will be about step four of the road map."
This was followed by the government's decision not to add any countries to its international travel "green" list – while also removing Portugal from it.
PHE, meanwhile, said Bolton remains the area worst affected by the Indian variant. Cases have risen by 795 in a week to a total of 2,149.
However, it added there were “encouraging signs” the transmission rate was beginning to fall following testing and vaccination drives in the area.
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