Christian apologist Ravi Zacharias has announced he has a rare form of bone cancer and has asked the Christian community to pray for relief from "horrific night pain".

In a post shared on Zacharias' Facebook on Thursday, he explained that he has a malignant tumour of the sacrum, a very rare cancer called Sarcoma.

The cancer was discovered after Zacharias developed severe pain following a surgery on his back three weeks ago. A biopsy was taken during the procedure, which revealed he had a cancerous tumour.

Treatments to reduce the tumour is expected to begin in two to three weeks, once Zacharias has made a full recovery from his back surgery.

Zacharias said he has faith that God is with him through this difficult news.

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

"We received literally thousands of messages from people all over the world saying you were praying. I have every belief God directed and prompted my surgeon to his discovery of this tumour. Margie and I and our family are so grateful for your continued prayers for the journey that lies ahead," he added.

He has asked Christians to continue to pray for him.

"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.

Zacharias has thanked his team for their "love and support" throughout his ill health and states he will suspend all speaking engagements until at least the end of the summer.