Since the Delta variant of coronavirus, first discovered in India, was detected in the UK in April, cases have surged across the country, with the variant now making up 96% of new infections.
Experts confirmed last week that the variant is 60% more transmissible than the Alpha variant, first discovered in Kent in 2020.
Scientists at the Wellcome Sanger Institute have used genomic sequencing to produce the following maps, which show how rapidly the Delta variant has taken over in England.
The darker the colour, the higher the proportion of cases caused by the Delta variant.
The maps show the proportion of cases in a two-week period up to 15 May, 22 May, 29 May and 5 June.
On 15 May, the Delta variant made up 38.6% of new cases in England. On 5 June that figure had leapt to 88.4%.
In some areas, the proportion of the Delta variant is estimated to be as high as 100%.
Data from the Wellcome Sanger Institute tracks community spread, and excludes cases in people who have recently returned from travelling.
The rapid spread has caused serious concern among experts and advisers to the government. It is understood that Boris Johnson will confirm on Monday evening that the next stage of lockdown easing, earmarked for 21 June, is being delayed by four weeks.
The government had intended to remove all legal limits on social contact in England from next week, but is expected to delay in order to allow time to vaccinate a higher proportion of adults.
On Friday the UK recorded 8,125 new daily COVID cases – the highest tally since February.
On Sunday, 7,490 cases were reported in the latest 24-hour period.
Watch: Boris Johnson faces Tory backlash over lockdown easing delay
The removal of restrictions would mean no rules on how many people you could invite into your home, while pubs, restaurants and cinemas would no longer need capacity limits.
Last week, Public Health England (PHE) said cases of the Delta variant had more than tripled.
As of last Wednesday, the UK had seen 42,323 confirmed cases of the B.1.617.2 variant, up 29,892 from 12,431 the previous week, an increase of 240%.
The BBC reported that ministers intend to keep the current rules in place for another four weeks after 21 June, up until 19 July.
At a press conference at 6pm on Monday, the prime minister will be joined by England’s chief medical officer Professor Chris Whitty and the government’s chief scientific adviser Sir Patrick Vallance.
Health secretary Matt Hancock is expected to make a statement outlining the plans in the House of Commons on 9pm on Monday.
Johnson is expected to face a backlash from many Conservative MPs over the planned delay, which will have to be voted on in the House of Commons.
Watch: 7,490 more positive COVID cases confirmed in UK