Distressing footage has emerged of dirty water being poured around a homeless man outside a train station.
The clip shows a man in a Southern Rail uniform repeatedly emptying a bucket of soapy, brown liquid around the man who is lying outside Sutton station in south London.
Three members of Southern Rail staff have since been suspended.
"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," the person who is believed to have filmed the video wrote on Facebook.
British Transport Police (BTP) said they were investigating the incident as common assault.
Communities Secretary James Brokenshire said he was writing to Southern Rail to ensure the "distressing" incident was dealt with "swiftly and effectively".
He added: "This shocking act shows a complete lack of empathy towards someone who is vulnerable and in need of help."
Rail Minister Andrew Jones said the incident was "absolutely unacceptable" and said he was pleased to see Southern Rail "swiftly" suspending the workers identified.
The footage shows a staff member swilling water towards the man's right side before walking to his left side.
He tells the man on the ground: "It smells. You've been here all morning. Get up and go."
The staff member then proceeds to swill more water on to and around the man.
The homeless man then sits up and talks to a second member of staff but their conversation is not audible.
A woman in the clip is heard saying: "That's absolutely disgusting. Disgusting behaviour of another human being."
Passenger services director for Southern Railway, Angie Doll, said the operator was taking the incident "extremely seriously" and that it had opened an investigation.
"We suspended the two members of staff seen in the video and a third colleague who we identified from station CCTV," she said.
"We would like to apologise to the homeless man and we are trying to make contact with him to offer help and support."