The water company scored 63 out of 100 in its service incentive mechanism (SIM) statistics for 2011-13.
After Thames Water, the companies to receive the lowest points were Southern Water and South East Water – both with 64 points.
Wessex Water was the highest scoring water and sewerage company with 85 points.
The industry average was 76.
Natalie Beckerman, customer service director for Thames Water, said: “We are not yet where we want to be, but we have made major improvements over the past year towards getting our customer service in a much better place.”
Ofwat will use the scores during the three years from 2011 to 2014 to penalise or reward companies during the price review next year.
The scores published on Friday are intended to provide the companies with information to factor into their business plan for 2015-20, which will be sent to Ofwat in December.
Penalties for companies could be as low as minus 1 per cent of their regulated turnover, while rewards could amount to an extra 0.5 per cent.
The scores are measured on a water company’s customer service performance annually, including the number of complaints they receive and how satisfied customers are with how their contacts are handled.
Each company is given a score out of 100 depending on how well they do. The companies publish this SIM score each year alongside other information about their performance.
According to Ofwat, water companies will be able to use the scores alongside its methodology statement to “draw their own conclusions about their relative performance and likely rewards or penalties”.