Cancerbackup will be transferred to Macmillan Cancer Support and two of its trustees, including current chairman Dr Maurice Slevin, will join the board of Macmillan Cancer Support.
The two charities hope the move will mean that cancer patients across the UK receive information and support more quickly. Information services will be delivered under the sub-brand Macmillan Cancerbackup and will include an improved telephone helpline with access to nurses, nutritionists and financial experts.
The charities will be talking to the Charity Commission about legally integrating, but were not able to confirm whether they will make use of the Charity Commission's new register of mergers.
"We are truly delighted to have agreed this merger," said Jamie Dundas, chairman of Macmillan Cancer Support. "Macmillan's ambition is to be able to offer the fullest possible support to everyone affected by cancer. High-quality information is a critical element of this and Cancerbackup's expertise in this field is in a class of its own."
He added: "Macmillan is committed to maintaining and expanding Cancerbackup's excellent work, and I am confident that together we will be able to do much more to help people with cancer."
Slevin said: "The combined strength of Cancerbackup's information and Macmillan's comprehensive services, and the fact that Macmillan is a household name, will vastly improve the delivery of cancer information.
"Together we are creating the opportunity to ensure that everyone in the UK affected by cancer will receive the highest quality information at the time they want and in the way that they want."