Chris Evans has announced he will be leaving the Radio 2 breakfast show after eight years at the helm, and 13 years at the station.
"I'm going to leave. I'm leaving Radio 2," he told listeners, but promised he'd stay on air until Christmas.
Explaining the decision, he said: "Some of us are mountain climbers [but] if you get to the top of your favourite mountain and you stay there, you become an observer.
"I want to keep climbing."
After the announcement, he played The Beatles' Twist and Shout, adding: "You have to keep shaking it up."
He later confirmed he was leaving Radio 2 to host Virgin Radio's breakfast show.
Evans took over from Sir Terry Wogan in 2010, and the breakfast show regularly attracts 9m listeners - more than any other UK radio programme.
He is currently the second highest-paid star at the BBC, earning £1.6 million per year.
The star and his wife Natasha are currently expecting twins; and he said his decision to change jobs had been "big news in our house over the past few weeks and months".
He admitted he'd been deliberating the decision for a year.
"A few weeks ago I made my mind up for sure," he said, adding he wanted to "start up on a brand new adventure".
Speaking to Father Brian D'Arcy, who was delivering Monday's Pause for Thought, Evans said: "As Sir Terry [Wogan] said: there's never a right time to go, but there could be a wrong time to go."
'Loved every second'
Evans first joined Radio 2 in September 2005 as the host of the Saturday afternoon show, moving to drive time in April 2006 and the breakfast show in January 2010.
During that time, he has also presented The One Show and the poorly-received relaunch of Top Gear following in 2016.
But it is his radio show, with its warm and energetic mix of interviews, music and trivia, where the broadcaster truly excelled, creating a familial atmosphere with segments like Kids Get a Fanfare and Listener Breaking News.
The children's writing competition 500 Words will remain part of the breakfast show after his departure, Evans said.
More from Chris on today's news... pic.twitter.com/SUwC9WSliR— BBC Radio 2 (@BBCRadio2) September 3, 2018
When is Chris Evans leaving?
The broadcaster revealed his decision shortly after 8:00 BST, after hinting at a "big announcement" throughout his Monday morning show.
The 52-year-old said he had "loved every second" of presenting the breakfast programme, but "I don't want this to be it".
He added that his final scheduled show was on Friday 21 December - but he'd like to bow out with a Christmas Eve special.
Virgin Radio has confirmed Evans will be waking up the nation on its radio station in the new year - although no official start date has been given.
Evans formerly hosted the breakfast show on the first incarnation of Virgin Radio - which went on to become Absolute Radio - between 1997 and 2001.
Virgin Radio UK, which was launched two years ago and is owned by Rupert Murdoch's Wireless Group, is a digital-only station with around 400,000 weekly listeners, compared with Radio 2's figure of 15 million.
Speaking about his new appointment, Evans said: "In many ways Virgin Radio is my spiritual home. I see nothing but exciting and groundbreaking opportunities ahead. In a medium that is changing so quickly on a daily basis, the potential for growth is unprecedented.
"Our plan is to give it all we've got, see where we can get to and have the most possible fun along the way. It makes me smile every time I think about it."
Richard Branson tweeted his happiness at the announcement of Evans' new post welcoming the star "back where he belongs on Virgin Radio".
Who will take over on Radio 2?
Although it is too early for his successor to be announced, Evans wished them well.
"Whoever takes over this show, good for you because it's the best. It's the best."
Fellow Radio 2 presenter Zoe Ball, who could be in the running for the slot, was among the first to react to his departure.
Flippin eck. CHRIS EVANS. You absolute legend. Always been such an inspiration, all learnt so much from you. Proper shocker on a Monday morning. You will be most missed Mr. Big big ❤️ https://t.co/oJle1MDsyV— Zoe Ball (@ZoeTheBall) September 3, 2018
Other Radio 2 names who could replace the star include Sara Cox and Simon Mayo.
"My prediction is it will probably be a female broadcaster," said Tony Blackburn, a former host of Radio 1's breakfast show.
"Whoever takes it over will have quite a job on their hands," he told BBC news. "The breakfast show is the most important show of the day."
Other broadcasters reacted with surprise to Evans' announcement.
Chris Evans you are amazing. You have taught me so much about broadcasting and will be beyond missed. Thanks for the music, laughs and constant inspiration. Huge love x— fearne cotton (@Fearnecotton) September 3, 2018
Chris Evans to leave @BBCRadio2. It has always been a brilliant show and he has presented it magnificently. Massive slippers to fill for someone.— Dan Walker (@mrdanwalker) September 3, 2018
In a statement, BBC director general Tony Hall said: "Chris has been an absolutely first class presenter of the Breakfast Show. He has brought both warmth and a genuine insight into what listeners want.
"He has given 100 percent to each of his BBC projects, including raising millions of pounds for Children in Need. I'd like to thank him for all his efforts over the years and wish him all the best for the future."