Rt Rev Peter Hancock, Bishop of Bath and Wells, has been diagnosed with acute myeloid leukaemia.
The disease is an aggressive cancer of the myeloid cells and requires immediate treatment
The diocese of Bath and Wells announced that Bishop Peter has stopped working and started treatment last week.
In a letter to the diocese last week, he said he faced a "challenging" few months.
"I have not been entirely well in recent weeks and on Friday I was diagnosed with acute myeloid leukaemia. I am really grateful to the doctors who I have been seeing and for the speed in which treatment is to be provided.
"I have informed my colleagues on the bishop's staff and I am confident that everything in the diocese will continue to run smoothly whilst I step back for a while. Jane and I have a wonderful family who have been very supportive and we are grateful to all our family and friends for their support, encouragement and prayers. I shall continue to pray for you all and would be grateful for your prayers.
"The Psalm at Morning Prayer today had the words 'The Lord is my strength and my shield; in him my heart trusts.' As I put my trust in the hospital staff, who I know will provide me with the most wonderful care, I am also trusting in the Lord, knowing that he too will watch over me in the days ahead."
Rt Rev Ruth Worsley, Bishop of Taunton, in Somerset, will be taking over his responsibilities.
She said: "May Bishop Peter and all of us know the peace of God which passes all understanding and may it keep our hearts and minds in the knowledge of His love, for each of us, daily."
