The regulator compiled annual performance indicators from all the water companies for 2012-2013.
The resulting analysis, published this week, showed Southern and South West Water at the bottom of the table, with three red scores each.
This indicates the companies’ performances were “significantly below target or expectation”, Ofwat said.
Northumbrian, Severn Trent, South West, Southern and United Utilities all scored red for internal sewer flooding. The scores reflected the number of incidents of internal sewer flooding for properties that have flooded within the last 10 years
South West Water said the number of internal sewer floodings in 2012-13 totalled 266. Within this total, the number of repeat internal floodings rose to 118 in 2012-2013 from 31 in 2011-2012.
In addition, both Southern and South West Water scored red for sewerage pollution incidents, and received a red score for “serious” pollution incidents. The pollution data was provided by the Environment Agency and relates to the 2012 calendar year.
Portsmouth Water was the only company to achieve a red score for leakage.
South Staffs, Sutton & East Surrey, Sembcorp Bournemouth, and Bristol Water were the only companies to register all of their indicators as” is in line with or better than expected”.
South West Water said it was “disappointed” by its environmental performance last year and that it “fell short of the high standards” it set itself on behalf of its customers.
A spokesperson for the company said: “This is why we are already implementing a region-wide plan, with the full backing of the Environment Agency, and are investing £85 million between 2010 and 2015 to significantly improve our performance.”
Southern Water said that flooding was a cause of pollution last year, with some of its sewer networks and pumping stations “overwhelmed by rain”.
Southern Water said: “We want to further develop our partnership with the Environment Agency and councils, which will take the lead on surface water management plans, to tackle the causes of flooding. We are on target to make significant improvements this year.”