Hundreds of additional hours were spent in ambulances caused by delays at Portsmouth Hospitals Trust last week, new figures show.
Across England, more patients waited in an ambulance for more than an hour than at any stage last winter.
Health charity the King’s Fund said the NHS is "bursting at the seams" as it recovers from the coronavirus pandemic and attempts to meet sharply rising demand, while think tank the Nuffield Trust said it is "painfully visible that ambulance services are under severe strain".
NHS England figures show 165 patients waited in an ambulance for at least one hour when they arrived at Portsmouth Hospitals NHS Trust A&E in the week to Sunday (December 4) – though this was down from 304 the week before.
A further 144 patients were forced to wait between 30 minutes and one hour, meaning 34% of the 898 total ambulance arrivals were delayed by half an hour or more, and at least 331 hours were lost.
NHS targets state trusts should complete 95% of all ambulance handovers in 30 minutes, with all conducted in less than one hour.
Danielle Jefferies, analyst at the King's Fund, said improving ambulance delays has been a government priority for some time.
Ms Jefferies added: "Problems at the hospital front door are indicative of issues at the back door.
"People are being stranded in hospital because of a long-term lack of investment in social care and NHS community services."
Nationally, 24,000 handovers (31%) were delayed by at least 30 minutes last week, while 11,300 (15%) had to wait more than one hour.
Both are significantly up on comparable December weeks in 2020 and 2021 and reached higher levels than any week last winter.
Sarah Scobie, deputy director of research at the Nuffield Trust, said: "NHS hospitals are desperately struggling to get patients in and patients out fast enough, and the situation continues to deteriorate as the temperature drops and we head into the most challenging winter months.
"Ambulances are one of the most visible and vital cogs of the emergency care machine, and it is becoming more painfully visible that they are under severe strain.
"The Government has confirmed additional funding to tackle the delayed discharges behind some of these problems, but it is far too late in the day to have a meaningful impact this winter."
Health and Social Care secretary Steve Barclay said the Government is providing £8 billion in funding "to boost performance and recover services to pre-pandemic levels".
Mr Barclay added: "The coming months will be challenging but I am determined to tackle waiting times and improve access for patients.
"We’re allocating an extra £500 million to speed up hospital discharge, getting ambulances back on the road more quickly, increasing the number of NHS call handlers, and creating the equivalent of at least 7,000 more beds."