Sir Richard Thompson says frontline staff are looking after so many patients they can miss signs that affect chances of survival

Care of hospital patients is under threat because overworked frontline doctors are looking after so many sick people that they are missing vital signs of illness that could affect chances of survival, one of Britain's most senior doctors warns today.

Hospital doctors are running around "like a scalded cat" trying to look after up to 70 elderly patients at a time, far more than the maximum of 20 regarded as necessary to ensure they receive proper attention, the president of the Royal College of Physicians, Sir Richard Thompson, told the Guardian.

Doctors specialising in acute medicine are so stretched they are not able to spend the ideal minimum of 15 minutes investigating each patient's symptoms because they have too many patients to get round in a typical seven-hour shift, he added.

In an interview with the Guardian, he said some doctors are facing caseloads during one shift of up to 70 patients, many of whom are medically challenging – what doctors call "multiply morbid", older people with simultaneous conditions such as heart problems, diabetes and breathing trouble.

"You try standing on your feet for seven hours trying to be on the ball, thinking of the various complications, being nice to patients, for seven hours. It's absolutely destructive. Not everyone has 70, but most people are looking after well over 20," Thompson, whose college represents most of the UK's 30,000 hospital doctors except those in Scotland, said Thompson.

"If you've got over 20 it becomes impossible. The care gets thinner and thinner. It means the consultant can't see the patient as much or indeed as early as they should do, so obviously the standard of care is going to fall".

Widespread understaffing, especially overnight and at weekends, is posing a direct threat to patients' safety, he warned.

Acute medicine consultants are supposed to see newly admitted patients within a few hours. "So you can imagine that something might be missed because someone hasn't been there to lead the team and to take a proper decision, so you can well imagine things going wrong" because they have not seen them quickly enough, Thompson said.

Doctors under constant "strain and stress" end up spending as little as five minutes with some patients. When that happens "yes, you miss things". Doctors increasingly do not have enough time to try to tease out the details of every patient's illness, he said.

In a strongly worded attack on ministers he accused the coalition of cutting the NHS budget despite repeated pledges, including from David Cameron, to protect it from the austerity programme. "In spite of what weasly words people at the top say, money's been taken out of the NHS." He cited the £2.8bn that has been given to social care in the past three years.

As a result, he claimed: "The NHS is under-doctored, under-nursed, under-bedded and under-funded. There are too few doctors to do the increasingly large job to a high standard, and safely, and compassionately."

A worrying number of hospitals, especially smaller ones, face "serious" gaps in their medical rotas, said Thompson.

Health secretary Jeremy Hunt's drive since the Mid Staffs scandal to ensure every patient has safe, high-quality care is unachievable because the NHS has neither the resources nor the workforce needed, Thompson said. It needs "billions" of pounds more a year to function properly, he claimed, and urged all political parties to be brave and accept that reality and start working out where extra funding should come from.

His broadside is the latest expression of growing frustration among bodies representing NHS staff at the tight budgets the service is under, including a £20bn "efficiency savings" drive, at a time of rapid growth in demand for healthcare.

The leader of the British Medical Association, Dr Mark Porter, said: "Many of Thompson's comments will be recognised by those working in the NHS. Doctors are working harder than ever before as all NHS services come under enormous pressure from a combination of rising workload, falling resources and staff shortages in key specialities."

He also backed Thompson's claim that the NHS budget has been cut. While ministers claim it has been ringfenced "in reality billions of pounds are being clawed back by the Treasury each year". Four years of pay cuts had left NHS staff feeling devalued and under attack", added Porter.

The chief economist at the Nuffield Trust health thinktank, Anita Charlesworth, said the coalition had fulfilled its promise to give the Department of Health an inflation-linked "flat real" increase during each year of this parliament.

But, she added: "It is clear that this is placing a growing strain on the NHS, most notably in the acute hospital sector. This is despite holding down the earnings of NHS workers. After 2015 it is not realistic to expect the NHS to continue to meet the demands of an expanding and ageing population within flat funding."

Hunt has also demoralised the service's 1.3m staff by "slagging off the whole of the NHS", Thompson added. "What he has done is emphasise too much the poor care instead of emphasising the good care" which most patients receive most of the time, he insisted.

A DoH spokesman responded that: "Patient safety and care is a priority for the government and it is right that we have high expectations for our NHS. While the NHS is one of the safest, most efficient healthcare systems in the world we should never shy away from trying to improve standards for patients."

The NHS now has the highest ever number of professionally qualified clinical staff since census records began, with plans in place to increase it further, he said, adding that there are now more than 7,500 more doctors working in the NHS than in 2010.