Six hundred and fifty jobs are to be cut by Thames Water as part of a plan to reduce costs.
The UK's largest water company, based in Reading, has told 350 members of staff they are at risk of redundancy.
It said a further 300 jobs would "potentially" be removed by not filling vacancies and through terminating contractor roles.
Thames Water said employees delivering services to customers would be largely unaffected by the changes.
In a statement, the company said most of the roles being cut were in support functions and management across London and the Thames Valley.
It said it was "focused on maximising every penny we spend on priorities including reducing leakage and customer complaints".
"This is clearly very difficult news for employees directly affected and we will do as much as we can to support them," the statement said.
'Disappointing' job losses
Thames Water was fined £120m last year for failing to fix leaks, and at that time apologised for letting customers down.
It recorded a pre-tax profit for the year to March 2019 of £51.6m, a 62% fall on 2017/18.
London regional organiser for Unison Jay Williams said it was "clearly disappointing when any utility company, especially one the size of Thames Water, announces any job losses".
He said the union would remain "engaged" with the employer to "restrict the number of posts at risk".
Thames Water's website says it currently has 15 million customers and more than 6,000 employees.