The British government said Friday that the new Delta coronavirus variant is 60 percent more transmissible in households than the variant that forced the UK to lock down in January.
The Delta variant, which first emerged in India, has caused a rise in cases in Britain, prompting questions about whether social distancing restrictions will be lifted as planned from June 21.
New research from Public Health England "suggests that the Delta variant is associated with an approximately 60 percent increased risk of household transmission" compared to the Alpha variant first identified in southeast England.
So far there have been 42,323 identified cases of the Delta variant in the UK, according to Public Health England's data, up from 29,892 on June 2.
The Alpha variant caused a surge of Covid cases in January before a mass vaccine campaign, leading to a three-month lockdown as hospitals were stretched to near-capacity.
The government has since ramped up its vaccine drive, and has now administered nearly 41 million first doses and nearly 29 million second doses to adults over 25.
This means 43 percent of the total population are fully vaccinated and 18 percent are half vaccinated.
But cases are rising again, with new daily infections hitting 7,393 on Thursday, a level not seen since February.
More than 90 percent of new cases were of the Delta variant, the government said.
However, the number of patients in hospital remains low, at just over 1,000 on Thursday, and Health Secretary Matt Hancock has said most in-patients are people who are unvaccinated.
The government said this suggested the vaccination programme is mitigating the impact of the Delta variant, urging the public to get both jabs.
Jenny Harries, chief executive of the UK Health Security Agency, said that "two doses provide significantly more protection" against the Delta variant than one.
The UK has reported 127,867 deaths from the virus, the highest toll in Europe.
Under the government roadmap, England plans to drop rules on numbers at social gatherings and allow large weddings and the reopening of nightclubs from June 21.
But officials have stressed that they are open to changing this date if the virus situation changes, with a decision due next week, as many businesses push for full reopening.