Donovan's show airs on Sunday evenings
Singer Jason Donovan broke broadcasting rules when he backed Boris Johnson to be London mayor, Ofcom has said.
The Australian star spoke out in favour of the Conservative candidate on his radio show, aired on 37 local stations.
Radio group GCap escaped with a warning after a listener to Kent-based Invicta FM complained to Ofcom.
Ofcom said the incident was a "very serious breach" of its rules, but added Donovan did not actively encourage listeners to vote for Mr Johnson.
Presenters are banned from showing any bias towards a political party before an election.
GCap said an apology had been read out on air, and it had taken measures to ensure a similar breach would not be repeated.
Donovan made the comments on his Sunday Night With Jason Donovan programme, aired on stations including BRMB in Birmingham, Red Dragon in south Wales and GWR in western England.
"It's time for a change. It's definitely time for a change. Boris Johnson. I have to say it. That's my political message," he told listeners on 27 April, just days before the election.
The show was not aired on GCap's London station, Capital Radio.
Ofcom said: "A presenter endorsing a political candidate at a time of an election is a clear and unambiguous breach of the due impartiality requirements of the broadcasting code.
"A presenter, albeit relatively new to his role as a radio presenter, was allowed to use his programme to promote an unchallenged political message in favour of a candidate for the 2008 London mayoral elections."
Earlier this year, Talksport fired presenter James Whale after he urged his listeners to vote for Mr Johnson, while the BBC warned 6 Music presenter George Lamb after he told listeners he would be voting for a candidate with "blond hair" on 1 May, the day of the election.