Coronavirus: UK cases hit four-month high for second day in a row
Saturday's figure is exactly 100 more cases than were confirmed the previous day, meaning both are the highest since 8 May.
A further 27 deaths of people with COVID-19 have also been recorded, taking the total to 41,759.
Experts caution more infections are likely to be picked up now because the number of coronavirus tests available has grown dramatically since the pandemic began.
New cases reported on 19 September in each nation:
England - 3,638
Scotland - 350
Northern Ireland - 222
Wales - 212
Fears are growing that daily infections could within weeks hit the figures seen in March and April if the government does not take action.
Ministers are thought to be looking at a temporary two-week "circuit break" in an attempt to break the chain of transmission.
The move could see pubs and restaurants ordered to close or face a 10pm curfew, with socialising between households banned.
Boris Johnson is facing pressure to call a meeting of the government's civil contingencies committee - better known as COBRA - which handles national matters of emergency but has not met since May.
Labour leader Sir Keir Starmer said the prime minister "can't afford to be slow", and Scottish First Minister Nicola Sturgeon added the meeting is vital "to seek as much coordination and alignment as possible".
Millions of people are already living under local lockdown, with many more possibly to follow after London mayor Sadiq Khan's warning it is "increasingly likely" fresh restrictions will be needed in the capital.
Meanwhile the testing system is still under strain, with people in hotspots complaining they cannot get a test or being told to travel hundreds of miles from their home.
It will take weeks to fix, Health Secretary Matt Hancock admitted, after the head of Test and Trace claimed nobody could have predicted the big rise in demand partially triggered by schools fully reopening.
A new rule was brought in from Monday to try to stem the growth of coronavirus making it illegal for people to gather in groups of more than six
But Professor Neil Ferguson - the scientist whose modelling led the government to order the first nationwide lockdown in March - said more measures will be needed "sooner rather than later".
"Right now we are at about the levels of infection we were seeing in this country in late February," he told BBC Radio 4's Today programme.
"If we leave it another two to four weeks we will be back at levels we were seeing more like mid-March. That's clearly going to cause deaths because people will be hospitalised."
