Sir Keir Starmer responded to a video of suspended Labour MP Mike Amesbury appearing to punch a constituent.
Footage appeared to show the 55-year-old MP for Runcorn and Helsby repeatedly hitting another man as others nearby shouted “Stop it”.
In a pre-Budget speech, the prime minister said: “I’ve seen the video footage. It’s shocking. We moved very swiftly to suspend him as a member and as a member of parliament.
“There is now a police investigation. In the circumstances, you’ll appreciate there’s not much more I can say.”
On Sunday, Cheshire Police confirmed a 55-year-old man had been voluntarily interviewed under caution in relation to the incident after officers were called to reports of an attack in Frodsham at 2.48am on Saturday.
“He has since been released pending further inquiries,” the force said.
