Starmer knows what the Conservative press did to Labour last time the anti-Semitism row got going, and he knows how long it lasted. I know several people who normally voted Labour who got into a 'no smoke without fire' mindset, thanks to the intensive attack the party was on the receiving end of.

I think he had no choice unless he was prepared to go through that again. I suppose, to be strictly accurate, he could have chosen to do that. It would not have been wise.