Almost 5,000 coronavirus cases have emerged that are suspected to be linked to a music festival in Cornwall earlier this month.
Health officials are investigating 4,700 cases after the Boardmasters event two weeks ago.
The cases are spread across the country but around 800 are living in Cornwall, a council official confirmed.
He said people who tested positive for Covid-19 reported they had been to the festival, held between August 11 and 15 in the Newquay area.
The all-ages festival, which included headliners Foals and Gorillaz, outlined its Covid-19 policy on its website.
It said the festival would ask all ticket-holders aged 11 and over to demonstrate their Covid-19 status through the NHS Covid Pass app before entering.
This meant attendees had to provide proof of a negative lateral flow test taken within 24 hours of arrival at the festival gates, proof of being vaccinated with both doses (with the second received at least 14 days before the festival), or proof of natural immunity following a positive PCR test (provided it had been at least 10 days and up to 180 days after taking the test).
People who camped at the festival had to take a second NHS lateral flow test during the event on Friday August 13 and log their results in the NHS Covid Pass app.
Attendees had to bring their own lateral flow tests.
Face masks were not compulsory but were encouraged.
Public Health England has said some people who tested positive reported they began developing symptoms after going to the festival.
Professor Mike Wade, deputy regional director and NHS regional director of public health for Public Health England South West, said: “We are working with colleagues at Cornwall Council to monitor what impact the Boardmasters Festival may have had on Covid case numbers, and we are starting to see cases reporting that they attended the event prior to developing symptoms or testing positive. We will continue to monitor the data closely as we have throughout the pandemic.”
He added that people should exercise caution when mixing and encouraged people to get vaccinated, test themselves with lateral flow tests before meeting friends and wearing a face mask indoors.
A spokesperson for Boardmasters said: “Since the Government allowed live events to return, we have worked closely with Cornwall Council’s public health team, putting in place risk management measures above and beyond national guidelines.
“These included use of the NHS Covid Pass as a condition of entry, which was introduced during the Government Events Research Programme earlier this year and is being recommended as best-practice at other large events.
“The system detected more than 450 people who would otherwise have been at risk of passing on the virus and as a result did not attend our Watergate Bay site or left the festival early. We are grateful to them and everyone else who took the extra steps this year.
“No event is able to eliminate risk entirely and the latest Test & Trace data includes reported infections among the 76,000 people who visited the festival or related activities at Fistral Beach, in Newquay and the wider area during the week of Boardmasters.
“We will continue to work with our public health partners to understand the extent to which attendance at the festival has contributed to the figures.
“We look forward to sharing our experience with our local authority partners and other large events so we can all continue to provide much needed economic benefit to our communities and entertainment to our loyal audiences.”