Large house parties attended by 16 to 19-year-olds were to blame for one of the biggest rises in coronavirus cases in England, a council said.
Hertsmere in Hertfordshire had the third highest rate of infection for any area outside the North West.
The borough council warned schools may close if residents do not “get a grip”.
Hertsmere, which has a population of just under 105,000, recorded 60 positive cases of Covid-19 in the seven days up to 4 September.
This gave the area, which includes Radlett, Bushey, Elstree and Borehamwood, 57.2 cases per 100,000 of the population.
Only Birmingham, Bradford and some areas of the North West had a higher per 100,000 infection rate.
The week before the 60 positive cases were recorded in Hertsmere, there were only 13 cases.
Tim Hutchings, the councillor responsible for public health at the council, said the rise in cases in Hertsmere was “significant”.
He said the outbreak was among a “group of young people in a number of large social gatherings in private homes in the last week of August”.
Conservative Mr Hutchings said the council had written to parents of children at schools where fellow pupils were self-isolating with a “stern warning: stop these house parties and private events or face enforcement action of up to a £10,000 fine”.