A wave of mutant ‘super flu’ is spreading through the UK with hospitals up and down the country declaring ‘critical incidents’.

Schools have been forced to bring back Covid-style firebreak lockdowns shutting their doors as hundreds fall sick, as the flu bug hits along with RSV and norovirus in what could be the worst outbreak in a decade, say experts.

The measures have been introduced as a mutant flu strain – known as H3N2 – is driving rocketing illness rates and filling hospital beds. The H3N2 subtype is one of three main viruses that cause seasonal flu. Experts warn this strain has mutated in a way that makes it both more severe and more contagious - and that it no longer closely resembles the version used in this year's flu jab.

There was an average of 1,700 patients in hospital with flu last week which is more than 50% higher than the same time last year.

At least two NHS Trusts have declared ‘critical incidents’, a term that’s used by hospitals when they can no longer guarantee patient care can be delivered safely. And this normally happens in response to overwhelming demand or infrastructure failure.

University Hospitals Birmingham NHS Foundation Trust, which covers Queen Elizabeth, Heathlands and Good Hope Hospitals along with nearby Solihull Hospital, has issued the alert due to “extreme pressures”.

The Trust posted on its hospitals’ social media sites saying A&E departments are facing huge demand. A Trust spokesperson said: “If you do not have an emergency or life-threatening condition you may be redirected to another healthcare service.”

And University Hospitals of North Midlands (UHNM) trust, covering both Royal Stoke University Hospital and County Hospital, Stafford, is telling people to only use A&E in a serious or life-threatening emergency.

Chief operating officer Katy Thorp said: “As anticipated, the pressure is greatest in our emergency departments, which are facing continued challenges as patients cannot be admitted to bed promptly."

Health leaders have warned there is 'no peak in sight' for the wave. National Medical Director for Urgent and Emergency Care, Prof Julian Redhead said: "The health service is bracing for an unprecedented flu wave this winter. Cases are incredibly high for this time of year and there is no peak in sight yet.

"The NHS has prepared earlier for winter than ever before, but despite that we know that ballooning flu cases coinciding with strikes may stretch our staff close to breaking point in the coming weeks."

Data shows the vaccine continues to offer protection against the virus - and health bosses have urged those eligible to get the flu jab ahead of what is expected to be an unprecedented flu wave this winter.

Dr Jamie Lopez Bernal, Consultant Epidemiologist at UKHSA: "With flu and RSV cases sharply increasing in the past week and Christmas just 3 weeks away, now is your last chance if eligible to help get protected before Christmas.

"We expect to see these viruses circulating more widely in the lead up to Christmas and more people getting sick - threatening a potential peak over the festive period and into the New Year .

"Nobody wants anyone in their family, especially young children, to spend Christmas in hospital. The best defence against this is a vaccine, but time is running out as it can take up to 2 weeks from vaccination for your body to build maximum protection. So don't delay any longer, if anyone in your family is eligible, book that appointment today or take your child to the local community clinic."