The warning came as Covid-19 infections in the UK fell to their lowest level for five months.
Analysis of Omicron BA.4 and BA.5 suggests they are likely to have a “growth advantage” over BA.2, which is the current dominant variant, according to the UK Health Security Agency (UKHSA).
“The reclassification of these variants as variants of concern reflects emerging evidence on the growth of BA.4 and BA.5 internationally and in the UK,” said Dr Meera Chand, UKHSA director of clinical and emerging infections.
“While the impact of these variants is uncertain, the variant classification system aims to identify potential risk as early as possible.
“UKHSA is undertaking further detailed studies. Data and analysis will be released in due course through our regular surveillance reporting.”
As of May 20, 115 cases of probable or confirmed BA.4 had been identified, with 67 in England, 41 in Scotland, six in Wales and one in Northern Ireland.
Another 80 cases of BA.5 were identified, with 48 in England, 25 in Scotland, six in Northern Ireland and one in Wales.
There is currently no data to determine the effect of the variants on hospital admissions in the UK.
Initial findings suggested BA.4 and BA.5 have a degree of “immune escape” — meaning the immune system can no longer recognise or fight a virus, which is likely to contribute to their growth advantage over BA 2, the UKHSA said.
A total of 1.3 million people in private households are estimated to have had the virus in the week to May 13, according to the Office for National Statistics.
This is down 14 per cent from 1.5 million the previous week and follows drops of 24 per cent and 32 per cent in the two previous weeks.
Total infections in the UK are now back to levels last seen in early December, when numbers had just started to increase because of the spread of the original Omicron variant.
Infections are now about a quarter what they were at the peak of the recent Omicron BA.2 wave at the end of March, when a record 4.9 million people were estimated to have Covid-19.