Peter Bradley, the national director for the ambulance services, told the Daily Telegraph the entire system was "struggling to cope".
Mr Bradley, who is also chief executive of the London Ambulance Service, said: "It has been the most difficult 10 days I have seen in the last 10 years. It is absolutely horrendous.
"Hospitals are full and A&E departments are struggling."
People are urged to make use of other health services, such as NHS walk-in centres, NHS Direct and pharmacies, unless their situation is genuinely an emergency.
In London, ambulance workers are handling 1,100 calls about "life-threatening" situations every day, a service spokesman said.
The total number of calls it is receiving every day has risen in the last few weeks from around 3,000 to nearer 4,000.
According to the service, the status of the situation has been elevated from "severe" to "critical".
In response, staff are working overtime, all non-essential meetings and transport have been cancelled and training staff have been put back on the road, the spokesman said.
In the West Midlands, 8,000 emergency calls were made last weekend, up 30% on the same weekend last year.
West Midlands Ambulance Service chief executive Anthony Marsh said these were "unprecedented levels" for the service, which covers cities including Birmingham and Coventry and the counties of Warwickshire, Herefordshire, Worcestershire, Shropshire and Staffordshire.
It's not the annual crisis that we used to know and love years ago
Health Secretary Alan Johnson
"All staff are working flat-out around the region to meet the extraordinary demand and are trying to do their best in very difficult circumstances," he said.
"Staff are tired and also sickness levels are increasing as they themselves become the victim of the norovirus and other stomach bugs that are around."
He said people were still dialling 999 for non-emergencies, meaning staff were arriving at people's homes to find they did not need the blue light service at all.
A spokesman added that at one hospital, Hereford County, over the weekend, he saw eight ambulances queued up outside waiting to off-load their patients.
He said it was a similar picture across the region.
Over the weekend, the North West service, which serves Merseyside, Lancashire, Cumbria and Greater Manchester, said the number of calls it had received so far this winter was higher than ever.
A spokesman for the Association of Professional Ambulance Personnel (APAP) warned 999 calls were increasing by between 5-7% per year, with many people calling for minor ailments which would be more appropriately dealt with by a walk-in centre or their GP.
Health Secretary Alan Johnson told the BBC that thanks to better planning, winter was no longer a time of crisis for the NHS.
"It's pressure we can cope with," he said.
[The NHS is] working very hard and very successfully to cope with very difficult circumstances, but it's not the annual crisis that we used to know and love years ago," he said.
A Department of Health spokesman said letters has been sent in October to all strategic health authorities and social services to remind them of the need to make sure winter plans were firmly in place.
Nigel Edwards, policy director at the NHS Confederation which represents managers, said: "Sustaining the current pace of work required for a prolonged period will be challenging. Nonetheless the system is coping."
The Tories warned NHS bed numbers were cut by 4% last year, and that bed occupancy rates are over 95% in some trusts.
Shadow health secretary Andrew Lansley warned pressures at the level seen during the last winter crisis nine years ago would be "immensely difficult" for the NHS to cope with."
Lib Dem health spokesman Norman Lamb said: "The health system is under impossible strain, putting staff under intense pressure and risking patient safety."
A spokesman for the NHS in Scotland said it was well-placed to cope with demand, and that the incidence of flu cases was lower than during an average flu season.
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