A birthday party held by young people has been linked to a localised spike in Covid-19 cases in August.
Cases in Arlesey, Shefford and Stotfold in Bedfordshire rose from 12 to 23 within a week of the party.
An investigation by Central Bedfordshire Council and Public Health England revealed its source, the Local Democracy Reporting Service said.
Most social gatherings of more than six people will be illegal in England from Monday, amid a steep rise in cases.
Central Bedfordshire councillor Tracey Stock said track and trace was used to link the cases in in the three villages to the party.
She told the council's Social Care, Health and Housing Overview and Scrutiny Committee that the "majority of cases were among young adults".
"Mobile testing capacity was brought into the area and a communications campaign targeted the young people," she said.
"That area has seen a real drop following the birthday party - hopefully that was a one-off incident."
The cases went from 12 in the week beginning 2 August, to 23 the following week.
Ms Stock said: "The data for the week beginning 24 August shows just three cases in those wards."
A third of all cases in England last week were among people aged between 20 and 29.
Health Secretary Matt Hancock has warned the UK risked a second spike if that age group does not follow social distancing rules.