He has no option. Tom Watson, Labour's Deputy leader and up until now, loyal to an echo, has told him that he has lost the confidence of the Parliamentary Labour Party.
Never mind that Corbyn is the darling of Labour's left wing activist base and their surrogate stalking horse 'Momentum'. If you lose the confidence of your parliamentary colleagues the activist base matters nowt ... sure you can hang tough and claim that you have a mandate from the wider Party; you can even call for a new election, stand and win, but it will be a Pyrrhic victory. A hopelessly divided parliamentary team and it's a recipe for electoral disaster.
The Tories too have their issues but I suspect they are savvy enough to sort out their leadership wrangles behind closed doors.
Hat-tip. Stick the name Dan Jarvis in your memory banks. Jarvis is 44 YO and MP for Barnsley Central. Ex Major in the Parachute Regiment, served in Kosevo, Iraq and Afghanistan, decorated. Tipped by Labour insiders as someone to watch and was, for a time, Shadow Youth Justice and Victims Minister under David Miliband. My contacts in the UK military speak highly of him.