James Whale had been with Talksport since it launched as Talk Radio in 1995
Radio host James Whale has been fired by Talksport after urging listeners to vote for Conservative candidate Boris Johnson in London's mayoral elections.
The comments were made on 20 March during his late-night programme.
A Talksport spokesman said: "James Whale's contract with the station has been terminated after a breach of the Ofcom Broadcasting Code."
Under Ofcom rules, presenters are banned from showing any bias towards a political party before an election.
The Talksport spokesman described the matter as "very unfortunate".
"We have investigated the matter and after serious consideration, the station reached the decision that there was a clear breach of the rules and that we had no choice but to terminate his contract," he said.
"James Whale is guilty of a gross error of judgement, which we found to be totally unacceptable.
"Talksport has acknowledged the seriousness of the complaints received, does not excuse the content of the broadcast and has told Ofcom that it has taken steps to prevent this happening again."
The matter was being investigated by Ofcom, he added.
The presenter's agent Stuart Hobday said Whale was "deeply disappointed".
"He's spent 13 happy years at Talksport," he said. "He's saddened at the way this has happened and it's come as a huge shock."
Ofcom have confirmed they are investigating the case.
A statement said: "Ofcom is currently investigating three complaints about comments made by James Whale during his show on Talksport.
"All UK broadcasters must adhere to the Ofcom's Broadcasting Code which sets standards for the content of television and radio broadcasting.
"In particular, broadcasters must ensure due impartiality is preserved on matters of major political or industrial controversy and major matters relating to current public policy."
Whale, 56, started his career at Metro Radio in Newcastle in 1974 and had his own late-night show on ITV in the early '90s, The James Whale Radio Show.
He had been with Talksport since it launched in 1995 as Talk Radio.
He considered standing for London mayor in 2006.