The iconic pop singer made the announcement Monday, saying he was retiring from concert touring upon receiving his diagnosis. The third leg of his 50th anniversary tour -- originally set to launch in March -- has been cancelled on doctor's orders.

Fans are said to be getting refunded in full.

Neil said in a statement ... "I plan to remain active in writing, recording and other projects for a long time to come." He added ... "My thanks goes out to my loyal and devoted audiences around the world. You will always have my appreciation for your support and encouragement."

Neil finished by saying ... "The ride has been 'so good, so good, so good' thanks to you."

He'll be presented with a Lifetime Achievement award at the Grammys this weekend.