Greg James has posted an emotional message to fans before he takes on “the biggest challenge of my life” and swaps slots with Nick Grimshaw to take over the Radio 1 breakfast show.

It was announced in May that Grimshaw will step down in September after a nearly six-year stint on the early-morning programme.

He will take over James’s drivetime show, and James will move up to the breakfast slot.

In an emotional Instagram post, James wrote: “If you listen to my show regularly, this is for you. I don’t want to get too sappy over this because I’m about to take on the most exciting job in the world but…A lot of life happens in 6 years and through all the shit it throws at you, good and bad, I’ve relied on you to provide brilliant bits for the show and if you’ve relied on me to bring some stupidity to your day, that makes me incredibly happy.”

He thanked his sidekick Chris Smith, calling him “the smartest man I know. Also one of the funniest.”

He added: “I’m sad we won’t be working together on the new show but I have loved every second with him on air. And have got a best mate out of it. There’s no one that gets my brain quite like him and it’s been a total pleasure to come up with stupid ideas with him.”

He said the pair would continue to write their Kid Normal books together, as well as work on “other secret podcast projects.”

He continued: “The next thing I do is going to be the biggest challenge of my life but I am so ready for it. And I hope you can join me on it. Bring on the nonsense.”

Paying tribute to Grimshaw, with whom he has been playing a national game of hide and seek, he said: “All my love to Grim who is going to be brilliant in the afternoons. It’s a show that has incredible listeners who love a laugh and it also has the best hours in the world, which I know he’ll love.

“He is insanely talented and it’s been a pleasure to spend loads of time with him recently working out this change over and the Hide and Seek madness.

“Me and him are gonna do a load more stuff like that in the coming months. Oh, I got sappy, sorry. We’re off to get drunk.”