The variant, known as B.1.621, was designated a variant under investigation on Wednesday.
There is no evidence to suggest it makes the vaccines less effective or causes more severe illness, PHE said.
Sixteen confirmed cases of B.1.621 have been identified across the country so far.
Most of the cases have been linked to overseas travel and there is currently no evidence of community transmission in the UK.
The strain had previously been designated a signal in monitoring.
PHE said it is carrying out laboratory testing to better understand how the mutations affect the virus’s behaviour, while contact tracing and targeted testing is being deployed to limit the variant’s spread.
The development comes as official figures showed coronavirus rates among people in private households in England were at their highest level since January, with one in 75 people infected. This is a rise from one in 95 people in the previous week.
The number of Delta cases – the highly transmissible variant first identified in India – have risen by 33,716 since last week, to a total of 286,765. The strain accounts for approximately 99 per cent of cases across the UK.
The coronavirus’s reproduction number, or R rate, in England remained unchanged from last week at between 1.2 and 1.4.
PHE’s data also showed that between 21 June and 19 July, 1,788 people were hospitalised after contracting the Delta variant. More than half (54.3 per cent) were unvaccinated, while nearly one-third (29.6 per cent) had received both doses of a vaccine.
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Dr Jenny Harries, the chief executive of the UK Health Security Agency said: “This most recent hospitalisation data shows once again just how crucial vaccination is in protecting us from severe illness and death.
“Two doses of vaccine are far more effective against Covid-19 than a single dose, so please make sure that you come forward to get your second dose as soon as you are invited.
“As we emerge from restrictions and vaccine coverage continues to rise, it is important to remember that while the protection provided by vaccination is excellent, it is not total. It is still as important as ever that we continue to exercise caution.”
A total of 3,692 people have been hospitalised with the Delta variant in the UK. Of this figure, 58.3 per cent were unvaccinated, while 22.8 per cent were fully vaccinated.
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