The NHS is starting to "creak at the seams" under the twin pressures of tighter spending and growing demand, the president of the Royal College of Physicians said yesterday.

Sir Richard Thompson was responding to figures showing that the number of patients facing long waits for hospital treatment increased by a third in the first year of the coalition government.

He was backed by senior medical figures who said there was increasing concern that the waiting time figures were moving in the wrong direction. Sir Peter Carter, chief executive of the Royal College of Nursing, said: "Once you take your eye off the ball it can take years to retrieve it. We are really worried about this."

Ministers insisted that long waiting times were under control, that the rises were small in the context of the millions being treated. Official figures show patients who waited more than 18 weeks for in-patient treatment, the legal maximum under the NHS Constitution, rose by 6,902 in the year to May 2011, from 20,662 to 27,564, a rise of 33.4 per cent.

Andrew Lansley, the health secretary, attempted to divert attention from the long waits yesterday by pointing out that the average in-patient waited 8.4 weeks for treatment, the same as a year ago.

The figures show 90.8 per cent of hospital in-patients were admitted within 18 weeks, just above the minimum 90 per cent threshold set by the Government to allow for people postponing operations for holidays and similar reasons.

More than 97.7 per cent of out-patients started treatment within the 18-week period. There were fluctuations from month to month but overall the proportion facing long waits for admission was "low and stable", Mr Lansley said.

Sir Richard said he was concerned about the implications of the rise. "[It] is both worrying for patients and evidence of an underlying cause – the increasing pressures on the NHS in general. The NHS is now creaking at the seams, with the growing number of patients needing urgent care, including older people with multiple health problems."