Reading-based Horseman Coaches confirmed that one of its vehicles was involved.
“Initial indications suggest that adverse road conditions, including ice, were a contributing factor,” its general manager David Elliott said.
“A full investigation is under way and we are cooperating fully with the relevant authorities and all parties involved.
“The safety and wellbeing of our passengers and staff is our highest priority. We are supporting those affected and will ensure that any learning identified through the investigation is acted upon.”
A Reading Buses spokesperson added: “We are supporting our driver after this incident and, of course, assisting the police with their investigation.”
Another two-car collision on the same slip road on Wednesday morning resulted in no injuries, SCAS added.