South Central Ambulance Service is the highest NHS spender on private services in the region, last year cashing out £16.3m.

Figures released under freedom of information laws revealed its spending was up from £13.6m the year before and £12.3m in 2014-15.

The NHS is spending more than £21million a year in the region to hire private ambulances to answer 999 calls, and take patients to hospital appointments.

A widespread shortage of paramedics and rising demand is behind the need to call on help from non-NHS providers, according to experts.

A spokesman for the Independent Ambulance Association (IAA) said the main reasons for a rise in private ambulance use in the last two years were “staff shortages in NHS ambulance trusts, combined with continued increases in demand”.

The benefits of using independent firms include flexibility and good value for money as “it’s cheaper for the NHS than paying overtime”, he added.

Private ambulances are hired from private firms as well as charities such as St John Ambulance and the Red Cross.

The figures also showed some trusts are having to spend more on certain groups of temporary staff, including emergency paramedics.

A Department of Health spokesman said: "The ambulance service answers more than 10 million calls every year and the vast majority of patients get an excellent service.

"Occasionally, ambulance trusts use other providers including St John Ambulance to help with spikes in demand, and these providers are subject to the same rigorous safety and quality inspections as NHS ambulances."