Chris Moyles will present his last Radio 1 breakfast show on Friday 14 September, with Nick Grimshaw taking over 10 days later.
Scott Mills will fill in for the week in between.
Grimshaw faces the possibility of inheriting an audience of less than seven million when he takes over the show after the latest figures showed it pulled in 6.9 million listeners in the second quarter of this year.
That is down from 7.1 million in the first three months of the year.
Moyles has lost around half a million listeners in the last 12 months, according to the figures revealed by audience measurement body Rajar.
He is handing over to Grimshaw after more than eight-and-a-half years fronting the flagship show and his final period at the helm will be covered by the next set of Rajar figures.
Grimshaw - known to listeners as "Grimmy" - has been with the station for five years.
Moyles is talking with bosses about a new programme for Radio 1, to which he is contracted until the end of next year.
He was already due to take a break to appear as Herod in a touring version of Andrew Lloyd Webber's musical Jesus Christ Superstar.