Comedy stars French and Saunders are to take over BBC Radio 2 for Red Nose Day.
The duo - who have previously been presenters for the station - will become "comedy controllers" as they oversee the network for the day on March 15.
And its sister station Radio 6 Music will also feature comedy names in the run-up to this year's Comic Relief, among them The Simpsons and This Is Spinal Tap actor Harry Shearer who will host a show. Kathy Burke will also front an hour-long programme.
On Radio 2, Jennifer Saunders and Dawn French will be appearing across the schedule on Red Nose Day with BBC bosses promising they will be "generally causing chaos and confusion".
Station controller Bob Shennan said: "What better way to celebrate Red Nose Day's 25th anniversary than to hand the Radio 2 reins over to two of this country's greatest comedy legends."
The station will also see Russell Brand back on the airwaves more than four years after he left the station in the the wake of the Sachsgate scandal.
The comic - who caused a furore in late 2008 as a result of his on-air antcis with Jonathan Ross - will be featured when the station broadcasts from his charity concert next month.
Brand has organised and will host the Give It Up for Comic Relief show which is being staged at Wembley Arena featuring stars such as Noel Gallagher, Noel Fielding, Kasabian and Jessie J.
Jo Whiley will host coverage of the show from backstage.
Burke will appear on 6 Music, standing in for Jarvis Cocker on Sunday, March 3, while Shearer will step in for the former Pulp frontman the following week, March 10.