Radio station Talksport has been handed a £20,000 fine after former employee James Whale publicly encouraged listeners to vote for Boris Johnson.
Media regulator Ofcom issued the fine after Whale "seriously breached" broadcasting impartiality rules.
The outspoken presenter was sacked from the station in May, after urging listeners to cast their vote in the London elections in favour of the Conservative candidate, breaching official broadcasting guidelines in the process.
Whale, who had presented phone-in shows for Talksport since its launch in 1995, made the comments during his late-night show on March 20th.
Though Mr Johnson ended the London mayoral election as the victorious candidate, Ofcom guidelines prohibit presenters from expressing political preference in the run-up to an election.
"Breaches of this nature have the potential to cause considerable harm to the democratic process by conferring an unfair electoral advantage on a particular candidate," said an Ofcom statement issued today.
It added that Whale is an "experienced presenter who was aware that he should not have been making such comments, and who used his programme in a deliberate and conscious way to promote one particular candidate for the 2008 London mayoral elections".
The BBC drama starring Aidan Turner returns to BBC One on September 4th.