The ambulance service for Hertfordshire spent more than £14 million on hiring private ambulances last year – more than double the year before.

The East of England Ambulance Service Trust spent the second highest sum on private ambulances of any ambulance trust in the country this year, despite only spending £6.6 million on the same emergency services in 2015-16.

Figures obtained by the Press Association through Freedom of Information requests show NHS spending on private ambulances has risen by more than a fifth in two years to reach more than £78 million.

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

Experts said soaring demand was behind the need for them, as well as problems moving patients through hospitals, which means NHS ambulances cannot be freed up.

Dr Taj Hassan, the Royal College of Emergency Medicine's president, said: "It is concerning that trusts are having to use part of their budget for private ambulances, and serves to highlight the current levels of demand emergency departments are facing.

"Under-resourced departments are struggling with overcrowding and 'exit block', when patients cannot be moved in a timely manner to a ward.

"This means patients are waiting longer to be seen and ambulances cannot offload patients quickly because there is simply no room for them.

"Ambulances then have to queue outside emergency departments for longer than should be necessary, delaying them from getting back out into the community and creating a need for private ambulances."