Sir Terry Wogan, 71, told listeners of the long-running Wake Up to Wogan that he would be stepping down at the end of the year.
He said that he would not be quitting the airwaves altogether and had an exciting new show lined up.
Making the announcement live on air, Sir Terry said: "Now, if you'll pardon me, I've a little bit of news of my own.
"If the mail is anything to go by, most of the listening population have spotted a report that next year I'm going to turn into Chris Evans.
"And I hate to tell you, but it's true. I was hoping to break it to you, my loyal listener, more gently.
"I wanted to be the first to tell you. It's the least I owe you, for endless years, countless hours of morning companionship, friendship, good humour, and laughter. Your loyalty and support has been a beacon of love in my life."
Sir Terry said the decision was the hardest of his broadcasting career.
He explained: "I'd rather leave while we're in love, as the song says, while the programme is the most popular on British radio, while we still delight in each other's company.
"And so we will, until the end of the year, when my good friend Chris Evans takes over. I know that you'll give him the same love and affection you've always shown to me."
Announcing his new radio project, he explained: "And in the new year, I'll be starting a really exciting new show, live from the BBC Radio Studio at Broadcasting House in front of you, as my live audience, presenting the very pinnacle of live music, artists, guests, and of course, you, your mail, your warmth, your wit.
"So, this is not goodbye, it's not even au revoir... As they used to say when I was a lad 'See you later alligator... in a while, crocodile'."
Sir Terry first joined the Radio 2 Breakfast Show in April 1972 when he was just 34, with his banter and observations of life quickly proving a huge hit with the station's audience. He was soon pulling in around seven million listeners.
However he left the show at the end of 1984 to take up his long-running stint on BBC1's three-times-a-week chat show Wogan which ran from 1985 to 1992.
He rejoined Radio 2 in 1993 and has presented Wake Up To Wogan since then.
Tim Davie, director of audio and music at the BBC said: "Terry is a legendary talent and I'd like to pay tribute, on behalf of the BBC and his millions of devoted listeners, to his unique place in UK broadcasting history.
His replacement, Chris Evans was gracious about the current host.
"This is very much Terry's story, not mine, as well it should be. To step down from something you have done so well and for so long and obviously still enjoy doing must be a tough call even for such a stoic as Sir Tel.
"That said, although I will miss the drive-time show which I have loved for the last three and a bit years, I couldn't be more excited at the prospect of hosting the flagship show on one of the BBC's national networks."
The move crowns a spectacular few years for Evans, who was sacked by the corporation 12 years ago.
Sir Terry, 71, has made it clear to friends he wants to slowly wind down his radio commitments, following his decision to cut back on television appearances last year.
The decision to retire from his weekday morning show will be sure to upset his army of fans, 8 million of whom tune in on a daily basis to hear his idiosyncratic musings, combined with pop music. No other radio show in Britain is as popular.
Chris Evans's career at the BBC has been marked by a spectacular fall from grace, followed by a very successful comeback. He rejoined in 2005 and now hosts the weekday drivetime show, which attracts five million listeners a week.
In 2006 Evans won the coveted Sony Music Radio Personality of the Year award and during his acceptance speech thanked the BBC for giving him a "second chance".
Evans, 43, was fired from his Radio 1 breakfast show in 1997 after he continually failed to turn up to work.
Copyright Speakers Corner 2016