Seriously ill patients are being kept in ambulances outside hospitals for hours so NHS trusts do not miss Government targets.

Thousands of people a year are having to wait outside accident and emergency departments because trusts will not let them in until they can treat them within four hours, in line with a Labour pledge.

The hold-ups mean ambulances are not available to answer fresh 999 calls.

Figures obtained by the Liberal Democrats show that last year 43,576 patients waited longer than one hour before being let into emergency units.

Dr Jonathan Fielden of the British Medical Association said: "The vast majority of patients coming into hospital by ambulance are in critical need of care in hospital and therefore delay can worsen their outcome."

But a shortage of out-of-hours GP care, after thousands of doctors opted out of treating patients outside working hours under lucrative new contracts, means more and more are going to casualty units, putting them under greater pressure.

"This is evidence of shocking systematic failure in our emergency services," he said.