The prime minister has described video footage that emerged of MP Mike Amesbury appearing to punch a man to the ground as "shocking".
Amesbury has been suspended from the party and has had the Labour whip withdrawn after the CCTV and mobile phone footage appeared on Sunday.
Sir Keir Starmer said the party had “moved very swiftly” to respond.
"There is now a police investigation and in the circumstances you'll appreciate there's not much more I can say about that," he added.