John Mulholland, former media editor of The Guardian, was fired after arguing with Mirror Group management about the way a delay in the project was handled last week.
Mr Mulholland is understood to have written to David Montgomery, chief executive of the newspaper group, stating that he could no longer work with Jeremy Reed, managing director of Sporting Life.
Mr Mulholland was furious that he was kept in the dark about a hiring freeze which preceded a decision last week to delay the relaunch of the newspaper.
The 150-year-old racing title closed earlier this year and was supposed to relaunch on 19 October as a general sports newspaper modelled on the Italian daily Gazzetta dello Sport.
However Mirror Group announced last week that it was delaying the relaunch until next year pending extra market research into the project.
Mr Mulholland had already offered 65 journalists jobs on the new newspaper; 21 of those had been offered posts while he was in the dark about the hiring freeze.
In a day of frantic activity following the return of Mirror Group chief executive David Montgomery from holiday the media group issued a statement: "MGN has terminated the employment of John Mulholland with immediate effect. His continued presence would further delay and put in jeopardy the whole project. In the face of misleading articles, the company wishes to reassure all those journalists employed on the Sporting Life project that it will proceed under new and professional leadership."
Richard Ellis, deputy editor of the newspaper, is understood to be staying with the project. Mr Mulholland failed to secure a meeting with Mr Montgomery to discuss the position of journalists who have resigned their old jobs and now face a delayed launch of Sporting Life. Before he could meet Mr Montgomery the statement announcing his sacking was issued while he was out of his office at lunch. The newspaper's skeleton staff had to contact Mr Mulholland to tell him the news after it appeared as a Press Association story.
A clearly bitter Mr Mulholland said: "Apparently I didn't have the required managerial skills demanded by Mirror Group. This comes as no surprise - it is in a league of its own when it comes to managerial skills.
"The people I feel most sorry for are the 65 journalists who agreed to join - leaving brilliant jobs - on the basis of the vision we outlined. The fact that I could not match David Montgomery's peerless managerial technique is, of course, a huge disappointment to me."Reuse content