Ofcom today fined Talksport £20,000 for breaching the Broadcasting Code after presenter James Whale encouraged listeners to vote for Boris Johnson in London's mayoral elections.
Whale was sacked after the edition of The James Whale Show was broadcast on 20 March this year.
Ofcom found that the licensee had "seriously breached" the due impartiality rules at the time of an election.
The watchdog said the presenter directly encouraged listeners to vote for Mr Johnson in the upcoming London mayoral elections and criticised the incumbent, Ken Livingstone.
A statement from Ofcom said: "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."
Ofcom has also directed Talksport Limited to broadcast a statement of its finding.
During the edition of the show, Whale took two calls from listeners and, along with his producer, discussed Prime Minister Gordon Brown and his backing of Mr Livingstone's candidacy for the 2008 mayoral elections.
Whale said Mr Livingstone "has been nothing but a complete and utter tragedy for the capital city".
The veteran presenter also directly encouraged listeners to vote for Mr Johnson, saying people would have a better quality of life under Mr Johnson's leadership.
He said they would not be "ripped off nearly as much, if at all".
Whale said that if people did not vote for Mr Johnson they only had themselves to blame if "Livingstone gets in for another term".
Ofcom received three complaints about the matter - and recorded a breach of Rule 6.1 of the Broadcasting Code.
The rule states that broadcasters must show due impartiality in their coverage of elections and referenda.
This is to help ensure that no unfair advantage is given to candidates through promotion in the broadcast media, irrespective of whether the candidate can be shown to have actually benefited.
Ofcom found: "The subjects of James Whale's comments - the 2008 London mayoral elections and which candidate to vote for - were matters of major political controversy...
"Alternative views about the London mayoral candidacy and Ken Livingstone's record as London Mayor were not represented in the programme so as to even attempt to achieve due impartiality.
"In any event, it is not appropriate for presenters to use their position to urge voters to support political parties or candidates.
"The presenter's comments were not simply a 'vote for...' comment, but amounted to a direct political message to his listeners.
"This effectively resulted in the programme becoming a platform in support of Boris Johnson and critical of Ken Livingstone."
The pending period of the London mayoral elections began on 18 March and the breach had the "potential to cause considerable harm to the democratic process", Ofcom found.
TalkSport carried out its own investigation and said in a statement previously that Whale's contract had been terminated.
The statement said that: "...there was a clear breach of the rules and that we had no choice but to terminate his contract.
"James Whale is guilty of a gross error of judgment which we found to be totally unacceptable."
Whale landed a drive-time slot for LBC 97.3 - working on the same radio station as Mr Livingstone.
Asked on the station's website how he felt about that, Whale said: "Absolutely fantastic because I believe in democracy and free speech and Ken is one of the funniest guys I've ever met..."Reuse content