Doctors and paramedics have told the BBC that long waits for ambulances across the UK are having a "dangerous impact" on patient safety.
BBC analysis found a 77% rise in the most serious safety incidents logged by paramedics in England over the past year, compared to before the pandemic.
In Wales, Scotland and Northern Ireland, the 999 system is also under "tremendous pressure", doctors say.
NHS England said the safety of patients is its "absolute priority".
In October, nine-year-old Willow Clark fell off her bike on a country path in Hertfordshire, cracking her helmet and leaving her with a fractured skull and a nine-inch laceration across her leg.
"I could see it was a really bad accident and I was 20 minutes away from home screaming for help," said her mother Sam. "These really nice people who were passing by phoned 999.
"They explained she had a severe head injury and her leg was badly hurt but we were told it would be a 10-hour wait for an ambulance and we'd have to get her to hospital ourselves."
When they got to A&E, Willow was immediately transferred to the trauma department. Doctors told her family that she should not have been moved because of her back and neck injuries.
She later found out that Willow had been classified as an "urgent" category three case, meaning an ambulance should have arrived within 120 minutes.
Coroners and lawyers have highlighted recent cases including:
Staffordshire's assistant coroner issued a 'prevention of future deaths' warning after a patient in Stoke died after waiting eight hours for an ambulance.
The family of a man who died after waiting nine hours for treatment has issued a legal challenge against the Northern Ireland Ambulance Service over a "chronic shortage" of ambulances.
The London Ambulance service is investigating after a man died when paramedics took almost 70 minutes to respond to a suspected heart attack.
Dr Katherine Henderson, an A&E consultant and president of the Royal College of Emergency Medicine, told the BBC's Today programme the problem with ambulance waits was "more serious than we've ever seen it".
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Source: BBC News, 12 May 2022