Health officials are braced for a bank holiday surge of coronavirus cases as about 500,000 people head to music festivals and millions more venture to the tourism hotspots.

Cases in Cornwall and Devon have reached record levels as infection rate among teenagers saw five-fold increase after the Boardmasters festival this month.

Although the vaccination programme is helping to suppress large numbers of deaths and serious illness, officials warned on Thursday that parts of the NHS are battling “unprecedented high-level demand” and that more cases could disrupt the return of schools next week.

The UK is expected to enjoy warm and sunny weather as it marks the first bank holiday weekend since most coronavirus restrictions were lifted this summer.

At least eight festivals are expected to attract about 500,000 people across England – including Leeds and Reading, Creamfields in Cheshire and Victorious in Portsmouth – with some events taking place this weekend having been postponed earlier in the year due to Covid. Pride celebrations are also taking place across the country, with tens of thousands of people expected on the streets of Manchester alone.

The south-west has the highest infection rate of any English region, with 46 of the country’s 50 Covid hotspots when looking at areas of about 8,000 people. The top three are all in Newquay, Cornwall’s party capital.

Officials believe that more than 5,000 infections may be linked to Boardmasters festival earlier this month, equating to about one-fifth of the region’s 26,000 cases in the week to Monday.

Rachel Wigglesworth, director of public health for Cornwall and the Isles of Scilly, urged tourists to take a lateral flow test before they arrive and only visit if they have a booking, after similar pleas by officials in the Lake District this week.

The lifting of restrictions and the successful vaccination programme had caused people to drop their guard against Covid, she said, adding to an “really high pressured situation” for Cornwall’s health system.

“Living with Covid is not the same as pretending it doesn’t exist, and I think that’s what has happened,” she said. “It’s not just younger people, that’s a general picture for people who have been told that all gloves are off and they’re taking that at face value”.

She added: “In Cornwall, we’ve always been trying to take a cautious approach because we’re expecting high visitor numbers. Our population has been cautious up to now. I think there’s a sense that everybody feels they’re on holiday now the restrictions have been removed.”

South Western ambulance service said on Thursday that it was responding to about 3,000 emergencies a day – 15% higher than last summer – amid “unprecedented high-level demand” due to high Covid rates and increased social activity. The army has been drafted in to work alongside paramedics in the region to help cope with the demand.

Soaring cases among mostly unvaccinated teenagers are driving the region’s rapid increase. The infection rate of 15 to 19-year-olds in Cornwall has jumped nearly five-fold in a week. Devon, Torbay and Plymouth have also seen similar rises among teenagers, pushing their infection rates up to among the highest in the UK.

The vaccination programme means hospitals are dealing with far fewer Covid patients than previous peaks, but this is alongside other emergencies and elective surgeries. The number of Covid patients in the south-west has nearly doubled in the past month, to 427 this week, although this is far below the peak of 2,347 on 20 January.

Dr Arif Rajpura, the director of public health for Blackpool council, said he expected the increased mixing over the bank holiday to lead to a rise in cases but that the bigger increase would happen when schools return from next week.

“I think it will result in a rise because of all the activity and festivals. The issue for us is the mixing among the young people who are then going back to school and it’s likely that may lead to an increase. Scotland is seeing something similar,” he said.

“That’s why we’re really emphasising to people: if you’re going to go out over the bank holiday, do it safely. Wear a face covering in crowded places, take a lateral flow test before you go, and demonstrate a little bit of social distancing.”

Torbay council said it was urging people to stay away if they had any of the wide range of symptoms associated with Covid, or had been in contact with a positive case.