The service is currently discussing plans to create a West Midlands-wide force. Roger Thayne said he stood down at a meeting on Thursday.

Mr Thayne had said he would resign if the merger went ahead and said he is now sure it will. A new head of the trust has already been appointed.

Mr Thayne, who declined to comment, will remain in his job until 31 March.

'Standards may drop'

He is also working in an advisory role for the Welsh Ambulance Service on an interim basis.

The number of ambulance trusts in England could be cut by two-thirds under government proposals.

Four ambulance trusts - Staffordshire, Coventry and Warwickshire, Hereford and Worcester, and West Midlands and Shropshire - could merge to create one trust under the plans.

Mr Thayne has previously expressed fears that standards may drop if one regional trust is chosen.

He said there were concerns about whether the system will still operate in the same way, if staff will be able to use their skills and if ambulances will be as well stocked and as fast to respond.