Three members of Southern Rail staff have been suspended after a homeless man was dowsed with dirty water outside a railway station on Sunday.
Footage posted online shows a man in a Southern Rail uniform repeatedly emptying a mop bucket of soapy, brown liquid around another man who is lying outside Sutton station in south London.
"This is how Southern fail staff treat another human being in Sutton and I didn't start filming till they had already wacked (sic) him with the mop and put even more water on and around him," wrote the person who is believed to have filmed the video, posting it to Facebook.
British Transport Police said they are investigating the incident as common assault.
Communities Secretary Rt Hon James Brokenshire MP said: "This shocking act shows a complete lack of empathy towards someone who is vulnerable and in need of help.
"I am writing to Southern Rail to ensure this distressing incident is dealt with swiftly and effectively."
The video begins showing a staff member in a Southern Rail uniform swilling the water towards the man's right side before walking round to his left.
"It smells. You've been here all morning. Get up and go," he says, before swilling more water onto and around the man laying prone on the ground.
The homeless man sits up to talk to a second member of staff, but the conversation is not audible.
A woman can be heard saying: "That's absolutely disgusting. Disgusting behaviour of another human being."
Angie Doll, passenger services director for Southern Railway, said: "We take this incident extremely seriously and have started an investigation.
"We suspended the two members of staff seen in the video and a third colleague who we identified from station CCTV.
"We would like to apologise to the homeless man and we are trying to make contact with him to offer help and support."
Rail Minister Andrew Jones said: "This behaviour is absolutely unacceptable and we have a zero tolerance approach to abusive behaviour across our network.
"I was pleased to see Southern Railway acting swiftly to suspend the two individuals concerned, and would urge passengers to report any incidents of this kind to the British Transport Police immediately."