The latest “big change” in coronavirus infections across the UK is of “great concern”, England’s deputy chief medical officer has warned.
Prof Jonathan Van Tam said people have “relaxed too much” over the summer and “we have got to start taking this very seriously again”.
If not, the UK will have “a bumpy ride over the next few months”, he warned.
He said that infections among younger people in EU countries had later filtered through to older age groups.
His comments came as more parts of the UK are to face tougher restrictions following a rise in the number of cases.
On Sunday UK authorities announced 2,988 new cases – the highest figure since 22 May, while a further 2,948 cases were reported in the 24 hours to 09:00 BST on Monday.
Stricter rules on home visits will be extended to two more areas in the west of Scotland from midnight.
In Wales, the county borough of Caerphilly is to be placed under a local lockdown from 18:00 BST on Tuesday.
“People have relaxed too much, now is the time for us to re-engage, and to realise that this a continuing threat to us,” Prof Van Tam said.
He added that hospital admissions and deaths were “at a very low level” in the UK and the rise in cases was most prominent among those aged between 17 and 21 – but the country risks following the trajectory of some EU countries.
“Where case numbers rise initially in the younger parts of the population they do in turn filter through and start to give elevated rates of disease and hospital admissions in the older age groups, and we know that then becomes a serious public health problem,” he said.
“The fact that 17 to 21-year-olds are not becoming ill means they are lucky, but they also forget because the disease is not severe for them that they are potent spreaders.”
Transport Secretary Grant Shapps has also announced travellers arriving in England from seven Greek islands will have to self-isolate for 14 days from 04:00 BST on Wednesday.
They are Crete, Lesvos, Mykonos, Santorini, Serifos, Tinos, and Zakynthos (also known as Zante).