People heading to Cornwall to celebrate New Year have been urged to pack their own first aid supplies as health services struggle under “extreme pressure”.

Royal Cornwall Hospitals NHS Trust, which has urged people to only call 999 or visit A&E for life-threatening illnesses and injuries, advised people to pack pain relief, flu and cold remedy and rehydration powders, as well as any prescription medicines.

The trust tweeted the advice: “Heading to #Cornwall this #NewYear? Just in case, be wise and bring these three self-care kings! Pain relief, flu and cold remedy and rehydration powders. And don’t forget to pack any prescription medicines, too. #HelpUsHelpYou.”

The trust attached an image detailing what people should have in a first aid kit, including bandages, dressings, tweezers, scissors, antiseptic and medical tape.

South Western Ambulance Service, which covers the region, has declared a critical incident due to being under “extreme pressure”.

On Wednesday morning, it said there were 482 patients waiting for ambulances, with 106 patients awaiting handover at hospitals across the region.

Adrian Harris, chief medical officer of Royal Devon University Healthcare NHS Foundation Trust, described how emergency departments were under “incredible pressure”.

“I’m asking all of the public to think very carefully before attending, to think about using 111 either online or on a telephone, to think about going to their pharmacy, and when necessary contacting their general practitioner,” he said.

“We are very, very busy so please don’t attend unless absolutely necessary. If you’re in doubt and you think you need help, please come and see us. We’re open but we are very, very busy.”

Other trusts to declare a critical incident include Surrey and Sussex Healthcare NHS Trust, which said there was “significant ongoing pressure on local NHS services”.

The trust has seen “record numbers” of people attending accident and emergency departments, calling 111, accessing GP services and calling 999, it said.

There are also “ongoing challenges in discharging patients who are well enough to leave hospital”, as well as an increase in staff sickness.

North East Ambulance Service declared a critical incident on 27 December, describing “unprecedented pressure across the health system”.

It said there were “significant delays” for more than 100 patients waiting for an ambulance, together with a reduction in ambulance crew availability to respond due to delays handing over patients at hospitals.