Gordon Brown will not be removed from office by his party, he will remain Prime Minister until the General Election - whenever he chooses to call it. The reason is really quite simple - no-one with any prospect of winning a leadership battle against him (even an outside chance) would want the job.
If you were an eager young Labour MP, keen to make your mark with a glittering future ahead of you would you be willing to be the person that led your party to the heaviest defeat it has ever suffered? If you were a grizzled old-stager seeing out the last years of your political career in comfortable semi-retirement as a Labour MP would you want your footnote in history to be that you led your party to the heaviest defeat in its history? If you were a mid-career ambitious Labour MP judging that you have the time to suffer this defeat and rebuild with yourself as leader would you be prepared to begin your golden reign by leading your party to the heaviest defeat in its history?
Perhaps, if you were a Labour Parliamentarian whose aims were to make society better, to improve lives and to save your movement and your party to fight another day you might consider a battle against Lord Mandelson for the levers of power. That, of course, would require rectitude, a degree of moral fibre, and a willingness to sacrifice your own betterment for the sake of the greater good. You're not going to find that in today's Parliamentary Labour Party.