Mr MacKenzie said he was leaving to form a consortium to bid for Talk Radio.

Roy Greenslade talks to Radio 5 Live: Rupert Murdoch has never lost his respect for MacKenzie (4'17")

But one expert told the BBC the move could be a smokescreen for a return to Sun publishers News International.

Mr MacKenzie said in a statement he had enjoyed a "terrific time" at Mirror Group, but this was a once-in-a-lifetime opportunity.

Mirror Group chief executive David Montgomery said: "Kelvin has done a fabulous job for the company.

"I am really sorry to see him go, he said, but he has a burning ambition for this project and we cannot stand in his way."

Mr MacKenzie joined Mirror Group three years ago as managing director of L!ve TV after leaving The Sun. Six months ago he was made deputy chief executive.

Topless darts

He was brought in to work the old magic with the ailing Daily Mirror, which like the other red-top tabloids, has been steadily losing readers to the middle-market Daily Mail.

His tenure may have been short, but figures released at the beginning of this month revealed that the Mirror is still losing readers, but at a slower rate than its great rival The Sun.

As editor of The Sun, Mr MacKenzie articulated the tabloid brashness of the 1980s.

With headlines like "Gotcha!" and "Freddie Starr ate my hamster", he became a legendary editor, boosting the paper's sales with a brash mixture of irreverent news coverage, royal exposes and topless girls.

At the rival Mirror Group, he relaunched the cable station L!ve TV with gimmicks such as the News Bunny and topless darts, which got the channel talked about, if not watched.

'Murdoch never lost respect'

Last week the man who took over from him as editor of The Sun, Stuart Higgins, also stepped down.

Former Mirror editor Roy Greenslade said he suspected the move may be a smokescreen for a return to News International and its proprietor Rupert Murdoch.

He said: "Rupert Murdoch has always admired Kelvin. Even when he left he never lost his affection for him and certainly never lost his respect.

"I just wonder if Kelvin is returning to Rupert and the whole Talk Radio thing is just a smokescreen.

"A very senior executive at News International said after the departure of Stuart Higgins that more surprises were to come and I just wonder if this is the surprise."