The annual hospital guide from health care information firm Dr Foster raises concerns that hospitals in England are under so much pressure that patient safety could be at risk, says Guardian health correspondent Denis Campbell guardian.co.uk

Hospitals are "full to bursting" and bed use is reaching such "dangerous" levels that staff are struggling to maintain the safety and quality of patients' care, claims an authoritative report based on the NHS's own performance data.

Bed occupancy rates are often well above 85%, the maximum for patients to be well looked after and not exposed to health risks, according to official statistics collated by healthcare information firm Dr Foster, which the government half owns.

The analysis also found that death rates at more than a dozen hospital trusts in England are "worryingly high". At 12 trusts, death rates were higher than expected on two out of four hospital mortality measures.

Senior doctors warned that the findings reflected their increasingly frantic efforts to find beds for the growing number of emergency admissions and that repeated breaches of the 85% limit mean that patients – especially the elderly – are being scattered across hospitals. A lack of beds in specialist wards results in patients being sen to wards where staff may not know how to look after them properly. Some operations being cancelled because of this "overspill", hospital doctors' leaders say.

The NHS recognises that occupancy rates above 85% make it difficult to give high-quality care, minimise the risk of hospital-acquired infections and avoid busy staff making mistakes with medication.

Ministers say hospitals across England do manage to maintain 85% average bed occupancy across the year and the service is flexible enough to cope well with fluctuations in demand.

But NHS figures published in the independent health information service Dr Foster's new Hospital Guide, which analyses the performance of every hospital in England in 2011-12, show occupancy was running at 88% in midweek throughout that period and averaged 90% for 11 of the 12 months, excluding the NHS's quiet periods, such as Christmas and new year.

The national level was over 85% on 230 of 365 days of last year (62.8%) and over 90% occupied on 19 (5.2%), NHS data given to Dr Foster shows. "The NHS is full," said Roger Taylor, director of research at the firm, which is co-owned by the Department of Health. Up to 29% of beds occupied by people who could have been treated elsewhere and other inefficiencies are costing the NHS billions of pounds a year, Dr Foster estimates. Over half such cases involve patients over 75 years old.

In addition, the NHS's Health and Social Care Information Centre says over the whole of 2011-12 many of the 146 hospital trusts it has data for, ran at 90% capacity or higher.

"Hospitals are busier than they have ever been and the beds that we have are under significant pressure", said Mike Farrar, chief executive of the NHS Confederation, which represents hospitals. The NHS in England has shed a third of its total stock of beds in the last 25 years, including 4,000 since 2007.

Hospitals increasingly declare themselves on "red alert", which means they have very few beds left, or even "black alert", when they have to shut their doors to new admissions and send patients elsewhere, said Dr Andrew Goddard, the director of the medical workforce unit at the Royal College of Physicians, which represents hospital doctors.

"If you ask any doctor in this country they would say that the system is straining to burst; particularly in winter, but now it's increasingly happening the rest of the year. Hospitals always seem to be full. There always seem to be problems findings beds for patients."

Dr Foster may underestimate the overcrowding, said Goddard. "The situation in the NHS is worse than this report portrays it. Their figures give bed occupancy rates at midnight. But hospitals are actually even busier between noon and the late afternoon, when we are trying to get patients home and deal with the new arrivals."

Shunting patients with conditions such as pneumonia on to surgical wards means they may not receive the specialist care they need, and hospitals lose income as procedures are cancelled because there are no beds for patients awaiting surgery, he said. "These things are happening across the country. That's pretty common and it's getting worse and worse".

Paul Flynn, chair of the British Medical Association's hospital consultants committee, said: "Most of us would recognise the picture of the NHS being 'full to bursting'. It is running at excessive occupancy for far too much of the year. It used to be that pressures were winter pressures but nobody calls them winter pressures any more because they run for most of the year."

In September the Care Quality Commission (CQC), the NHS regulator in England, took legal action against Mid Yorkshire Hospitals NHS Trust when it found that some patients who were not awaiting surgery were being left for as long as four days in the day surgery unit at Pinderfields hospital in Wakefield, where they had no washing facilities, no lighting and no bedside storage because the unit was generally used by those staying for less than a day.

Dr Foster's findings pose a challenge to the coalition's attempts to defend its stewardship of the NHS at a time when the service is trying to cope with a £20bn efficiency drive and rising demand for its services. But Jeremy Hunt, the health secretary, insisted that "The NHS is not overcrowded. On average there are around 20,000 of its beds available. Of course this goes up and down, but the NHS has practice and experience in managing peaks of demand, particularly in the winter."

Katherine Murphy, chief executive of the Patients Association, said: "These distressing figures reveal bed occupancy rates are at the very limit of what is safe or indeed desirable for patients."

Dr Foster's analysis of NHS mortality data shows 12 hospital trusts had worryingly high death rates, according to its measurements, that three had consistently high rates as judged by the Hospital Standardised Mortality Ratio indicator and death rates at weekends were high at five other trusts, possibly because too few staff were on duty.