Image caption An open letter calls for chief executive of the South West Ambulance Service to resign

Ambulance staff have called for their boss to quit as they "struggle to maintain a crumbling service".

GMB union members from South West Ambulance Service (SWASFT) have written a letter "apologising" to the public for "potentially putting them at risk".

They have told chief executive Ken Wenman government cuts have led to "despair and frustration" among staff.

Mr Wenman said SWASFT was working to "improve resource levels" and "urged" GMB to "re-engage and talk to us".

He has not commented publicly on the call for him to resign.

The open letter was addressed as an "apology to our families, friends and the community".

To the public, they said they were "sorry for not getting to you or your loved ones quick enough because there are just not enough of us".

They also apologised to family and friends for times when they missed "yet another family occasion".

They also wrote that they felt "unsupported" by their employer SWASFT.

Analysis by Matthew Hill, BBC West health correspondent

This dispute is all about changes to rotas as well as concern from members that they are having to work for longer than their usual 12-hour shift.

But it must be remembered the GMB is not recognised by SWASFT, and part of their mission is to recruit more members to take them above the 25% figure that would help that come about.

Having said that, the main union Unison is also concerned about work load, especially with the extra demands on their service due to problems with the out of hours service in Somerset, and closure at night of Weston A&E unit.

But Unison has not gone as far as to call for any heads to roll.

Gary Palmer, from the GMB, said: "We felt this recent letter on behalf of a group of GMB members particularly summoned up the general despair and frustration many staff currently feel from working within a service and role they love."

Tony Fox, from SWASFT, said: "We accept that there is always more to be done and we will continue to work closely with our colleagues and listen and respond to their needs."

The South Western Ambulance Service NHS Foundation Trust covers Cornwall, Isles of Scilly, Devon, Dorset, Wiltshire, Gloucestershire, Bristol, Somerset and South Gloucestershire.