Severn Trent has agreed to buy Dee Valley Water for about £78.5m.
The company has now suspended talks on an alternative takeover bid with the investment firm Ancala, which had bid roughly £71.3m.
Severn Trent said it would invest in the firm and help vulnerable customers but MPs in the area have raised concerns about the takeover.
Wrexham MP Ian Lucas said: "Dee Valley Water has been a small and quietly efficient company, which has gone about its business serving our area well in the past few years.
"It is now facing a takeover by a multinational company and I am concerned that profits, rather than service, will be put first."
Clwyd South MP Susan Elan Jones said it was "vital that reassurances are given about people's jobs and that local people's concerns are listened to - and acted upon".
Severn Trent played down the concerns.
A spokesman said: "Customers will share in half of any wholesale cost efficiencies achieved, which will be reflected in future bills."
A water company which supplies about 260,000 customers in north east Wales and Cheshire is being sold.