Ravi Zacharias has asked for prayer after being diagnosed with a rare form of cancer. 

The apologist's doctors made the discovery after he underwent spinal surgery last month that left him in "very severe" pain. 

The pain has been so crippling that he has been unable to sleep at night. 

While it had been assumed that this was a consequence of the surgery, he said on Facebook that a further biopsy revealed a malignant tumor on his sacrum. 

He said that treatment to shrink the tumor will begin in around four to five weeks after his back surgery has healed. 

While he had hoped to return to ministry in a few weeks, he now thinks it will be summer at the earliest before he is able to resume his speaking engagements.

"We are trusting the Lord in this, and we believe we have already seen evidence of His hand. For example, the tumor did not show up on any previous scans and was only discovered by my surgeon identifying it during surgery," he said. 

He thanked well-wishers and supporters for praying for him these past few weeks, saying he and his wife Margie had received "literally thousands" of messages from around the world. 

"I have every belief God directed and prompted my surgeon to his discovery of this tumor. Margie and I and our family are so grateful for your continued prayers for the journey that lies ahead," he said. 

He concluded by asking people to pray for relief from the pain he continues to experience at night. 

"We are trusting the Lord for His purpose. Please do also pray that God will take away this horrific night pain, which is the most difficult part of waiting," he said.

"In the meantime, I want to express my heartfelt thanks for supporting the ministry as it goes forward around the world unabated. We need you to continue to stand with us."