Listeners were left baffled by the erratic behaviour of Radio 2 presenter Ken Bruce today - until it emerged the show was being hosted by comedian Rob Brydon.
Brydon managed to get through almost the entire programme before eventually dropping hints that it was an April 1 wind-up.
Listeners took to message boards wondering whether Bruce was drunk.
Brydon had still not owned up by the end of the show when he handed over to the next presenter Jeremy Vine, who he called Tim - the name of Vine's comedian brother.
Some listeners were fooled by the switch. One tweeted: "Ken Bruce has been really annoying this morning totally unlike him but I for one am not impressed. There, I've said it now, rant over."
Others saw through the gag and praised Brydon's appearance.
And the funnyman himself dropped a hint during the show, when he posted a message on Twitter which said: "Loving Ken Bruce on Radio 2 this morning..."
Brydon repeatedly burst into incomprehensible rambles and cracked ribald jokes, far removed from Bruce's usual patter.
And at one stage he chatted to afternoon presenter Vine and grumbled he was "boring".
The programme also included a spoof interview in which Brydon interviewed Sir Terry Wogan - another impostor who turned out to be Peter Serafinowicz.
The real Bruce said after the show: "It's been wonderful to be able to listen to the radio - tuning into someone entertaining in the morning for a change. I'm offering him a contract to share the honours on a regular basis."
And Brydon said: "They were big shoes to fill but I loved the challenge of being Ken for the whole show. I'd seen Being John Malkovich and wanted to take it to the next level. Happy April 1, everyone."
