Hundreds of patients have had hospital appointments cancelled while others are being resuscitated in corridors as British hospitals shut down in the extreme heat. 

Doctors have set out the 'unacceptable' impact the heatwave is having on the NHS, with vital equipment including MRI scanners failing as the temperatures rise. 

The hot weather - with temperatures reaching 36C this week in parts of the UK - has also prompted a surge in the number of A&E visits as elderly patients struggle to stay cool. 

Several trusts have declared critical incidents as a result.

The Norfolk and Norwich University Hospitals NHS foundation trust, raised the alarm on Wednesday after its MRI scanners stopped working as a direct result of the extreme heat. 

The trust said at least 362 outpatients appointments had been cancelled and it was prioritising inpatients and those on suspected cancer two-week wait pathways. 

Magnetic resonance imaging - or MRI - scanners are used to help doctors see what is happening inside the body, and are often used to diagnose brain tumours and injuries. 

Without them, hospitals risk becoming 'settings where extreme heat puts patients at risk, rather than places of safety,' Dr Hllary Williams, clinical vice president of the Royal College of Physicians, warned. 

'Patients are facing overcrowding, and there are issues with machines, laboratories and kidney dialysis - all fundamental to providing safe patient care.  

Speaking to The Guardian, she continued: 'The impact of heatwaves cannot be overstated. There is a sense of foreboding when we see the weather forecast, because we know what is to come, and there is very little staff can do.' 

Other hospitals have reported similar problems during the heatwave. 

In Portsmouth, the Queen Alexandra hospital declared a critical incident on Wednesday after its cooling units failed, resulting in 'elevated temperatures' across the hospital. 

This has led to 'significant disruptions across several of [the trust's] services', including operating theatres, diagnostic scanning facilities and cardiac laboratories.

Meanwhile, NHS Hampshire and Isle of Wight said last night that soaring temperatures have put the entire health system in the region under immense pressure. 

It said the critical alert issued by the UK Health Security Agency this week indicates a risk to life, including among people who are normally healthy, prompting the trust to trigger measures to maintain services. 

The Royal Devon and Exeter Hospital followed suit, declaring a critical incident on Thursday evening 'due to the widespread nature of impacts the extreme heat and humidity' were having on both staff and patients. 

The London ambulance service also reported its highest number of life-threatening emergencies in a single day in its history.

Ambulance crews responded to 642 urgent calls on Wednesday - those related to the most serious, life-threatening injuries and illnesses such as cardiac arrest.  

The numerous incidents underline the pressure extreme weather conditions can place on hospital infrastructure, making an already precarious situation even more urgent. 

NHS workers say both temperatures and tempers are rising, as healthcare professionals struggle to deliver the level of care expected of them in the face of rising demand and workforce shortages. 

Resident doctors are now looking to specialist doctors to advise them on what to do in such unprecedented conditions and calling on the NHS to plan proactively, offering additional support to staff. 

'As clinicians, there are immediate steps we can take to protect patients, such as reviewing medications that affect hydration and ensuring they are stored properly,' Dr Williams added. 

'But this must be matched by system level change - strengthening infrastructure and embedding resilience into NHS reform.' 

In response, the Department of Health and Social Care said all NHS trusts are expected to have measures in place to deal with extreme heat. 

A spokesman added: 'Trusts are urgently coordinating response to increased demand on health services and protecting vulnerable patients.'