Hospitals were last night accused of keeping thousands of seriously ill patients in ambulance 'holding patterns' outside accident and emergency units to meet a government pledge that all patients are treated within four hours of admission.

Those affected by 'patient stacking' include people with broken limbs or those suffering fits or breathing problems. An Observer investigation has also found that some wait for up to five hours in ambulances because A&E units have refused to admit them until they can guarantee to treat them within the time limit. Apart from the danger posed to patients, the detaining of ambulances means vehicles and trained crew are not available to answer new 999 calls because they are being kept on hospital sites.

Last night the practice was condemned by doctors and ambulance union leaders and was described as a 'scandalous distortion of practice' by one MP. Dr Steve Field, chairman of the Royal College of GPs, called it 'absurd, entirely inappropriate and unacceptable'.

They were backed by Sam Oestreicher of the union Unison. 'A fully equipped ambulance and a fully trained ambulance crew are effectively babysitting patients when they should be out there dealing with emergencies. Ambulances should not be used as mobile waiting rooms. They should be freed up to do their job.'

Evidence of patient stacking is revealed in the official 'turnaround time' data from seven of England's 11 regional ambulance services who responded when asked for the figures last week. These show that delays of at least an hour are widespread in the NHS. Figures relating to the past 15 months show that a total of at least 44,000 delays were reported by the seven ambulance services.

In London, there were 14,700 occasions last year when an ambulance took at least an hour from its arrival at one of the capital's 35 hospitals to hand over a patient and be ready to respond to the next emergency. This figure includes 332 that took more than two hours.

The Department of Health says an ambulance should arrive in 15 minutes and, although it includes time taken to clean and restock a vehicle after a patient has been handed over, ambulance staff say that takes only five or 10 minutes.

'These figures show there's a terrible and colossal waste of ambulance resources going on in many parts of the country,' added Oestreicher, whose union represents about half the 30,000 ambulance personnel in England. 'The... problem is that A&E units aren't admitting patients who are in the back of ambulances if at all possible if it's going to compromise the four-hour target that they are set by the government to treat all patients in A&E. They are deliberately keeping patients outside waiting in ambulances.'

The London Ambulance Service admitted last month that 'long turnaround times at hospital did have a clinical impact as it meant patients were waiting for an ambulance'. It has recently held a series of 'handover summits' across the city, involving crews and A&E medical personnel, to try to reduce turnaround times by five minutes each and thus boost the service's availability of vehicles.

Norman Lamb, the Liberal Democrats' health spokesman, said the accusations represented 'a scandalous distortion of practice to meet a target that is meant to improve the service'. Lamb said that he would be writing to Alan Johnson, the Health Secretary, to demand an urgent investigation.

Dr Jonathan Fielden, the chairman of the consultants' committee at the British Medical Association, said: 'Undoubtedly some patients' care will have suffered as a result of these delays. The vast majority of patients coming into hospital by ambulance are in critical need of urgent care in hospital and therefore delay in getting to that critical care can worsen their outcome. That could include patients with heart attacks, certain types of strokes, breathing difficulties or trauma.'

The BMA claims A&E units are under pressure because of cutbacks in the number of hospital beds.

A Department of Health spokeswoman said last night that 'these statistics are based on only seven out of 11 trusts and measure the time taken to turn around an ambulance for its next emergency, including cleaning and restocking the ambulance ready to go back out on the road. They do not reflect time spent by patients in the ambulance before being admitted to accident and emergency. These figures must be seen in the wider context of the 4.3 million patient journeys undertaken by emergency vehicles in 2006-07.

'But it is clearly important that patients are handed over to A&E as soon as possible after the ambulance arrives.'