Amateur ambulance crews are being sent to 999 calls because of a shortage of professionals.

Sussex Ambulance Service NHS Trust today admitted volunteers from St John Ambulance and the Red Cross had been attending category-A emergencies in circumstances which were "not appropriate".

The staff shortage has been attributed to high sickness levels, which trade unions say are linked to stress. Up to 15 staff are also off work on suspension.

Unison branch secretary Duncan Jones said: "We are aware the volunteer reserves have been sent out to category-A emergency calls to provide back-up for the ambulance crews.

"Some members have told us they have attended these calls without the support of ambulances, which is obviously of concern."

He said individual volunteers were also called in to provide cover by riding with crews in trust ambulances.

Mr Jones said the practice was "a major problem".

A trust spokeswoman said the reserves, trained to a higher standard than most volunteers, had been increasingly called upon.

She said: "Reserves have been used more often because we have been receiving an increased demand for services and sickness levels have been very high.

"There have been occasions when they have been used for calls they should not perhaps have been used for. That has been recognised as not being appropriate."

She said new guidelines had been drawn up during negotiations with trade unions stating reserves should only respond to category-A emergencies in exceptional circumstances.

She added: "They may attend if they are in the area but it will be as well as, not instead of, our vehicles."

The new guidelines also state individuals should not work in trust ambulances, except during major incidents.

Earlier this month, The Argus reported the trust had been criticised by Government inspectors for a culture of bullying and harassment.

Trade unions Unison and the Association of Ambulance Personnel have since been involved in discussions to resolve a series of concerns about working conditions.

The use of the volunteer reserve crews responding to the most serious emergencies is one of the major complaints.

Janet Pattisson, the chief executive of Sussex St John Ambulance, said the county's 25 reserves were trained to a similar standard as ambulance technicians.

She said: "We only go to incidents at the request of the trust. I am not aware of any incidents which have caused concern to our members."

Talks have also prompted the trust to establish a whistle-blowing policy on bullying, which will become operational next month.

It is also reviewing its suspension policy and trade union recognition agreement.

A study will be held into the effectiveness of standby cover, when ambulances are positioned in remote roadside locations to respond to nearby incidents.