Fears have been raised that Southern Water could raise charges to customers by 73% by 2030.
The plans to increase bills to £759 a year, taking into account inflation, emerged in leaflets from a focus group run by the company.
Southern Water said the options outlined in the leaflet were part of a research programme which would be submitted to industry regulator Ofwat in October.
It said the figures did not include discounts of at least 45% to around 125,000 households in financial hardship.
In the leaflets it commits to reducing sewage spills by 25% before 2030 and 75% by 2050. The company has also pledged to invest an extra £50m in the next two years to reduce storm overflows.
Campaigners have branded the plans as 'a disgrace'.
Katy Colley, from Hastings Boycotts Southern Water, said the company had years to make the necessary investments from the money they collect from bills.
'Instead they paid out millions in dividends, executive salaries and bonuses, and now they want us to pay all over again for the works they should have already done.'
Southern Water provides water to 2.6 million customers and wastewater services to 4.6 million customers across Kent, Sussex, Hampshire and the Isle of Wight.
In 2021 it was fined £90m for dumping billions of litres of raw sewage into the sea.
Katy Taylor, chief customer officer of Southern Water, said: 'We know our communities want to see us investing to improve our environmental outcomes and to do it wisely, but we also recognise the concerns about rising payments in the face of a cost-of-living crisis.'
