The number of people infected with coronavirus in the UK has risen by as much as two-thirds, the Office for National Statistics (ONS) says.
It estimates that around 100,000 people tested positive in the week to 29 May, or one in 660 people - up from 60,000 the previous week.
A growing proportion looked like they were the Delta variant, first detected in India, the ONS said.
Infections were rising most steeply in England and Wales.
A further 6,278 confirmed cases in the UK were announced by the government in official figures on Friday, with 954 people in hospital with Covid and 11 deaths recorded.
This equates to a large increase in cases from a low base, but hospital admissions and deaths are down slightly compared to last week.
Increased testing for the so-called Indian variant, or Delta variant, as it's been named by the World Health Organization, in hotspot areas may mean this spike looks larger than it really is.