The water company is closing part of a lane of the main road in Bridgtown in a bid to establish why water has been flooding the road.
The work will be taking place near the railway bridge and Holiday Inn on Watling Street from Monday.
Traffic can still travel westbound on the A5, but a section of the eastbound carriage will be closed for potentially up to a week.
A full diversion will be in place including cars being directed to use the A460 from Junction 11 of the M6, Severn Trent said.
The company is investigating its sewer network in partnership with Highways England.
Carol Askey from Severn Trent said: "We’re carrying out this essential investigative work, so we can determine where the flooding on the highways is coming from, and understand if there’s an issue on our network or not.
"To do this work safely and to keep our teams and others safe, we will be working with the local council and highways to close a section of a lane on the A5.
"It will be our absolute priority to make sure the work is carried out as quickly as possible, with everything back to normal and open as soon as we can."