The UK has reported 5,274 new COVID-19 cases in the latest 24-hour period - the highest number of daily infections since late March, according to the government's data.
There have also been another 18 coronavirus-related deaths.
Thursday's COVID-19 cases are up by 944 on Wednesday's 4,330 - and deaths are up by six from yesterday's 12.
It is the highest number of infections since 26 March, when 6,187 were reported.
The new figures take the total number of infections recorded since the start of the pandemic to 4,499,973 and the total number of deaths to 127,812.
It comes as Public Health England (PHE) revealed the number of cases of the Indian (Delta) variant have almost doubled in the past week - rising by 5,472 to 12,431.
PHE officials said on Thursday that the Delta variant has now overtaken the Alpha (Kent) one as the most dominant in the UK.
They warned that there is "early evidence" to suggest the Delta variant carries a greater risk of hospitalisation, but that extra measures taken in Bolton - where cases are highest - appear to have successfully bought transmission rates down.
The decision not to add any countries to the UK's green travel list - and to move Portugal from green to amber - was down to concerns over a Nepal mutation of the Delta variant.
Portugal's positivity rate has "nearly doubled since the last review" following the spike, Transport Secretary Grant Shapps said.
Separately on Thursday, the number of people who have received their first dose of a COVID vaccine reached 39,758,428 and second dose 26,422,303.
In the latest 24-hour period 172,763 first doses were administered and 349,019 second doses were also given out.
It came after the government revealed more than half (50.2%) of UK adults have now been fully vaccinated with both doses.
On Wednesday, it was announced that three-quarters of all over-18s have received a first jab.