The UK is probing a new strain of the Delta Plus coronavirus variant which is feared may be the fasting growing yet.
A subvariant called AY.4.2 was designated as a "Variant Under Investigation" by the UK Health Security Agency on Friday.
The agency said there was some evidence that the variant could be more transmissible than Delta, Mirror Online reports.
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"The designation was made on the basis that this sub-lineage has become increasingly common in the UK in recent months, and there is some early evidence that it may have an increased growth rate in the UK compared to Delta," UKHSA said.
"While evidence is still emerging, so far it does not appear this variant causes more severe disease or renders the vaccines currently deployed any less effective."
However, the agency said so far it does not appear this variant causes more severe disease or renders the vaccines currently deployed any less effective.
It comes as government scientists have told Boris Johnson he needs to act now with Plan B measures if Britain wants to reverse surging Covid cases as we go into winter.
Advice to the Prime Minister and his Cabinet has been published in minutes of a meeting of the Scientific Advisory Group for Emergencies (Sage) on October 14.
Ministers were warned that "earlier intervention would reduce the need for more stringent, disruptive, and longer-lasting measures".
Sage evidence showed that Government advice to work from home would be the most effective measure, compared with mask-wearing and the introduction of Covid passports.
Despite rising Covid confirmed cases hitting 50,000 and expected to double in the coming weeks, ministers have refused to enact such Plan B measures instead insisting the pressure on the NHS is "sustainable".
The Sage document stated: "Modelling suggests that the stringency of measures required to control transmission of a growing epidemic is increased by a faster doubling time.
"In the event of increasing case rates, earlier intervention would reduce the need for more stringent, disruptive, and longer-lasting measures."
