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.
But the ONS data, which is not skewed by how many people come forward for tests, shows a genuine rise, with the proportion of people in England and Wales testing positive almost doubling in a week. In Wales, this is from a low bar so the ONS said it was only the "early signs" of an increase in cases.
The picture in Scotland and Northern Ireland is less clear - although Scotland has seen significant growth in infections in recent weeks.
At the end of May, the number of people infected was:
- one in 640 in England, compared with one in 1,120 the previous week
- one in 1,050 in Wales, compared with one in 3,850
- one in 800 in Northern Ireland, compared with one in 820
- one in 680 in Scotland, compared with one in 630
Delta variant dominates
While the ONS does not conduct genetic testing on the samples it uses to track the virus, it found a growing proportion looked like they were the Delta variant.
The test used to work out if someone has coronavirus looks at three of the virus's genes, one of which cannot be detected in the Kent - now named the Alpha - variant.
However a rising number of the ONS's samples did test positive for that gene, suggesting it is most likely the Delta variant (but could also be the Beta or Gamma variants found in South Africa and Brazil).
This chimes with genetic testing (known as sequencing) published on Thursday which suggested the Delta variant was now the dominant strain in the UK. It accounted for 79% of cases confirmed by laboratory analysis in the last week.
Health Secretary Matt Hancock said he couldn't "yet conclude with any confidence that there's an impact on your risk of hospitalisation," with the Delta variant.
But he added it was "vital to protect people at home, and play our part in making sure the world gets vaccines".
'Too early to say'
Paul Hunter, a professor of medicine at the University of East Anglia, said: "With the information available it is too early to say for certain whether the Delta variant does indeed cause more severe disease needing hospitalisation or not.
"What we can be confident about with available data is that if you have had one or preferably two doses of vaccine you are rather less likely to catch either Alpha or Delta variants, and even if you do become infected you are less likely to need hospital admission."
The virus's reproduction or R number - an estimate of how many extra infections each coronavirus case will lead to - is between 1 and 1.2 in England, meaning for every 10 people infected, another 10 to 12 more will catch the virus.
In Scotland the R number is estimated to be between 1.1 and 1.3.