Ambulance chiefs have clashed with Labour over a claim that private firms respond to 50 per cent of 999 calls in West Yorkshire on busy days.

Andy Burnham, Labour’s health spokesman, said the revelation would shock local people and raise fears that crews would lack vital skills and equipment.

During a Commons debate on the A&E crisis, he told MPs: “It shows that there is no part of our NHS that won’t be put up for sale by this Government.”

But the claim was dismissed by Yorkshire Ambulance Service NHS Trust, which insisted the use of private companies was “negligible”.

Insisting the proportion never reached 50 per cent, a spokesman said: “As a professional ambulance service, we would always prefer to respond to patients using our own staff and vehicles. However, when demand for our services is particularly high, our own resources can be complemented by St John Ambulance, British Red Cross and private ambulance service providers.”

Insisting the use of private operators was “common practice”, the Trust said there was no suggestion of any threat to patient safety.

He added: “The organisations we work with are all registered with the Care Quality Commission, the independent regulator of all health and social care services in England.”

Mr Burnham said he was alerted to the growing use of private ambulances after evidence was given to an emergency A&E ‘summit’ the party staged. Debbie Wilkinson, a paramedic in Leeds and chairwoman of the union Unite’s national ambulance committee, had described the pace of the increase as “insane”. And she said: “We usually use them at Christmas and New Year to help with the pressure – but we’ve just continued to use them. Some days, as many as 50 per cent of 999 calls are being answered by private ambulances. We can’t guarantee that who is turning up on your door is exactly who they should be.”

Seizing on that allegation, Mr Burnham asked: “Is the Secretary of State satisfied that these crews are appropriately trained and have the right equipment? Is there not a danger that, because contractors are operating in isolation from the rest of the system, they will too often just carry to A&E?”

No minister responded during the debate, but the department of health said, in a statement: “Using a variety of healthcare providers for patient transport services has been done for a number of years.

“Contracts to deliver patient transport are decided locally, and should be based on what is needed to meet patient demand.”

An arm of Arriva, the bus firm, has taken over ambulance patient transport services in Manchester – and Unite believes Yorkshire could be next.