Ambulance crews say more volunteers could be sent on 999 calls because of staff shortages.

Sussex Ambulance Service has used volunteers from St John Ambulance and the Red Cross to help cope with demand during weekends.

The volunteers are highly trained and part of a reserve team of workers called in when the service is at full stretch.

Unions say unless money is spent on getting more staff and extra vehicles on the county's roads, the volunteers will be needed more regularly.

An ambulance spokeswoman said: "We had a problem with short-term staff sickness over the weekend.

"This, combined with the good weather and the fact we are getting busier as we hit the summer, meant there were times when we needed extra support. The volunteers have all had extra training and are backed up by ambulance crews."

She said the service had strict guidelines on when volunteers could be used and they were more highly trained than most volunteers.

If the volunteers are in the area and they can get to the scene quickly, they are used. They are closely followed by ambulance crews.

Unison representative Duncan Jones said: "I have been told by colleagues that the reserve crews were needed over the weekend. This has happened before and there are concerns that unless we get more resources on the road to cope with staff shortages then this is going to happen more often."

St John Ambulance has 25 reserves who are trained to a similar standard and go to incidents when asked to by the trust.

A spokeswoman said: "The reserves are approved by the Government and we are called out when the trust is particularly busy or has any shortages."

The ambulance service has about 1,000 staff, more than half of them paramedics or ambulance technicians.

It deals with more than 400 emergency calls every day, rising to more than 600 during peak times.

During the last bank holiday weekend it received more than 1,800 calls.