* Increase to come into effect from April 1
* Highest average bill still South West Water
LONDON, Feb 5 (Reuters) - Household water and sewerage bills in England and Wales are set to rise by 3.5 percent or around 13 pounds ($20.46) over the next year, the water industry regulator Ofwat said on Tuesday.
The increase will take into account an inflation rate of 3 percent and will mean a total average bill of 388 pounds in 2013-2014, the regulator said.
The changes will come into effect on April 1 and apply until March 31, 2014, and bills will vary for each household depending on their water supplier and whether they have a water meter.
Ofwat's chief executive Regina Finn said she understood there was huge pressure on household incomes driven by inflation.
So far, however, average bills have stayed broadly in line with inflation and are 10 percent lower than what companies asked for.
"Back in 2009, companies wanted bills rises of 10 percent above inflation. That didn't chime with what customers told us they wanted, so we said they could only increase bills in-line with inflation," Finn said.
"We will make sure customers get value for money, and if companies fall short in delivering their investment promises, we will take action," she added.
Ofwat data showed the highest water and sewerage bills in the country continue to be from South West Water, with households paying an average 499 pounds even though bills will fall by 7.3 percent after the government promised contributions to reduce each household's bill by 50 pounds from April.
Among other suppliers with the highest bills were Wessex Water, which will leave households with an average bill of 478 pounds or an increase of 4.9 percent, and Southern Water, which will leave customers with an average payment of 449 pounds or a rise of 5.3 percent.
Thames Water will see the biggest percentage rise in water and sewerage bills of 5.5 percent, leaving households with an average bill of 354 pounds.
Britain's largest listed water provider United Utilities will see bills rise 3.4 percent, while those at Severn Trent will increase by 2.2 percent.
The higher bills will contribute to investment programmes worth around 25 billion pounds between 2010 and 2015, Ofwat said.