Trained volunteers have clocked up nearly 300 hours between them in the hunt for missing Thomas Edwards.

Boats, a helicopter and specialist dog teams are also engaged in the extensive search. The police’s specialist search and rescue team POLSAR is being helped by Wiltshire Search and Rescue (WILSAR), which is made up of people aged 18 and upwards from all walks of life, professionally trained to search and mobilised by text when required.

An SOS was first sent out to the team of 45 at 5.20pm on Sunday.

Chairman of WILSAR Gail Leaman said: “Within 15 minutes there were 20 people available, not working.

“When we go out, what we are looking for first and foremost is a person. “We just want to find him safe and well and return him to his family.”

They work for about six hours each day before fatigue sets in and they have to stop.

WILSAR has been called out about 60 times in Wiltshire in the past year. The chairman said this was among the longest searches they had assisted with.

Since Sunday they have been scouring the area from Studley Crossroads up to Chippenham. On Tuesday evening POLSAR turned its attention to the fields next to Pewsham Way. They were joined by specialist dog teams from South Wales today as they continued to search the Chippenham and Derry Hill areas, while specially trained officers from Avon and Somerset examined the River Avon in boats.

Police say the public have respected their polite request yesterday to leave the search element to trained bodies. A spokesman said: “It is good intentioned but could impact on how quickly we can work if somewhere has already been disturbed.”