The chief executive of the Oxfordshire and Buckinghamshire primary care trust cluster has resigned, HSJ has learned.

It is understood that Sonia Mills will step down from the trust in what is thought to be the first resignation of a cluster chief executive.

HSJ understands the resignation was unexpected. Ms Mills joined NHS Oxfordshire in March 2010 and became cluster chief executive in April this year. The cluster covers over 1.1m people in the South Central region.

A cluster spokeswoman said: “Sonia will be leaving her position as chief executive of Oxfordshire and Buckinghamshire NHS Cluster. She has spent some 35+ years in the NHS with a long and successful career in many parts of public service. Leaving the NHS is a significant personal decision for Sonia; she will be pursuing new challenges and vocation in the future.

Chair of Buckinghamshire and Oxfordshire Cluster Fred Hucker said “Sonia has been chief executive of NHS Oxfordshire since early 2010 and for the Buckinghamshire and Oxfordshire Cluster over the past six months. She has contributed significantly to the health system throughout both counties and has established a sound basis for the current transitional activities within the NHS nationally. She will be missed within our two counties and on behalf of the Board and all parts of the health system; I wish her well in her new endeavours.”