The government has reported the highest daily number of coronavirus cases in more than a month - as a quarantine for arrivals from Luxembourg was announced.
A total of 846 new cases of COVID-19 were reported on Thursday, the highest UK total since 28 June (901 cases).
A further 38 people have died across all settings after testing positive for coronavirus, taking the official death toll to 45,999.
The 14-day self-isolation requirement for arrivals from Luxembourg has been reintroduced across the UK from midnight on Thursday following a rise in cases in the country.
It comes days after the UK reintroduced the same restrictions for those returning from Spain.
The decision has provoked an ongoing row, with Transport Secretary Grant Shapps saying the government "had to act".
In the wake of the move, Prime Minister Boris Johnson has warned there are signs a "second wave" is surfacing in Europe.
Health Secretary Matt Hancock has echoed this, telling Sky News earlier he is "worried" about another spike of infections, with a "second wave starting to roll across Europe".
Ireland reported 85 new cases of the virus on Thursday, its highest daily total in more than a month.
Meanwhile, health authorities in France reported 1,377 more cases on Wednesday.
This brings the rolling seven-day average in France above the 1,000 threshold for the first time since May.
In the UK, it was announced on Thursday evening that separate households will be banned from meeting indoors from Friday in Greater Manchester, East Lancashire and parts of West Yorkshire after a rise in coronavirus cases.
Earlier, Boris Johnson said coronavirus is "bubbling up" in up to 30 areas across the UK.
He said levels of COVID-19 are "going down" in Leicester - the first city subject to a local lockdown - but Britons have to be wary of a "really damaging second wave".
"It's absolutely vital as a country we continue to keep our focus and our discipline and that we don't delude ourselves that somehow we're out of the woods or that this is all over because it isn't all over," the PM warned.
The places with the highest case rates per 100,000 people according to NHS Digital's latest data are Blackburn with Darwen (85.3), Leicester (57.7), Oldham (53.1), Bradford (44.9) and Trafford (39.3).
Behind them are Calderdale (32.4), Rochdale (30.9) and Sandwell (27.5) - which has set up its own contact-tracing system in a bid to contain outbreaks and out of frustration with the one run by Whitehall.
The full list of no-quarantine countries for English holidaymakers is below - while the list of Scottish exemptions can be found here: