Thames Water and Southern Water have rated poorly in handling customer complaints.
That's according to a new report by the Consumer Council for Water. Both companies saw an increase in complaints while others saw a reduction in 2019/20.
Two thirds of water companies saw the number of complaints from customers reduce in the same period, with fewer households having to turn to the consumer watchdog, the Consumer Council for Water.
Thames Water and Southern Water were the only two companies to rate poorly across all of CCW's complaint handling performance measures.
Whilst Bournemouth Water led the way as one of the four best performers.
The CCW revealed that complaints made in writing by customers increased by almost 10,000 in 2019-20 - a 13 per cent rise fuelled largely by a substantial increase in billing-related problems encountered by households in the Thames Water region.
Thames Water and Southern Water were the only two suppliers to perform poorly across all eight comparative measures used by CCW in the report, including complaints received in writing and those escalated to the consumer watchdog.
The Chief Executive of the CCW has written to both companies calling for an urgent improvement and has offered to support them in their efforts to tackle the root causes of their poor performance in 2019-20.
The total number of written complaints made to water companies reached their highest level for four years at 84,649, but the total was skewed by the poor performance of Thames Water.
It was hit by an additional 12,619 complaints - up 57% - to mark it out as one of the industry's worst performers for the second successive year.
Across the industry, problems with bills, including disputes over how much water a household has used and the recovery of debt - continued to cause customers the biggest headache, accounting for almost two-thirds of complaints.
Thames Water was the worst performer for these type of complaints with the company citing the rollout of its new billing system and an IT issue in the final quarter of the year for the sharp increase.
CCW was also disappointed to see Southern Water lose some of the good progress it had previously made after a 22 per cent increase in written complaints. The company also lagged behind the rest of the industry when it came to resolving complaints at the earliest stage of the process.
In a statement Southern Water said, "We fully recognise that we need to improve the service we deliver to our customers, reducing their need to contact us and complain.
"Though complaints increased over 2019/20, complaints have reduced overall by 68% since 2015.
"Our Customer Service Improvement Programme, launched in April 2020, will deliver a better experience to our customers across a number of areas. Since then we've seen a reduction in written billing complaints by 18%.
"We're also using insight into our complaints to ensure we're focusing on the changes our customers want to see, and are fully committed to improving our position."
Kelly Macfarlane, Thames Water customer experience director, said, "We recognise this is not acceptable and we are not happy with our performance last year. We always aim to provide great service for our customers and we're absolutely determined to improve our performance.
"We've been working with CCW over the past year and welcome their offer of continued support in identifying and resolving the issues causing complaints. We're committed to working together and listening carefully to our customers to improve their experience. This already has company-wide focus and is our top priority.
"We're making progress, despite the challenges of the ongoing pandemic, and since last year we have completed a huge programme of work to move all of our 3.6 million households to a new online billing platform. Whilst we recognise this has caused some issues and we are very sorry to any customers we've let down, we've now put the foundations in place to better support our customers.
"Once optimised, I'm confident the capabilities of this new system along with changes to the way we work will help deliver the step change we need to continually improve the service we offer customers and reduce complaints."
Thames Water also said that they offer lower tariffs for low income families, and are aiming to have 300,000 customers on its Waterhelp tariff by 2025.
They also confirmed they had launched a flexible payments scheme for customers who've experienced a significant financial impact as a result of the coronavirus outbreak.