Water charges for Scottish households are to rise for the third year in a row.

From April, water charges will increase by 1.6%, about £6 a year for the average household, Scottish Water said.

It means the average Scottish Water household charge in 2015/16 will be £346.

The company said that since 2009 its charges have reduced by 10% relative to the rate of inflation and are within price limits set by the Water Industry Commission for Scotland based on a new six-year agreement between Scottish Water and the Customer Forum.

Customer charges will help pay for a £3.5 billion investment by Scottish Water in the next six years which will further improve drinking water quality, protect the environment and support the economy and jobs in the construction sector, according to officials.

Scottish Water chief executive Douglas Millican said: "Scottish Water continues to provide one of the UK's best-value water and waste water services.

"We have listened to the views of our customers in setting our plans for the next six years.

"Our customers expect us to build on the significant improvements we have made to water services for the benefit of all our customers in Scotland as we embark on our new, six-year regulatory period.

"Our customers have told us that they value certainty in their charges over the medium term.

"Today's announcement signals stability in water charges for the coming years and provides certainty for customers.

"It follows a major customer consultation exercise and extensive discussion with customers and stakeholders, including the Customer Forum for water, to agree a 2015-21 business plan which is focused on meeting customer priorities.

"Charges allow us to maintain investment - ensuring we continue to provide fresh, clearer drinking water for our customers while protecting and enhancing the environment and supporting the Scottish economy.

"It means that, by 2021, household bills will have fallen further in real terms - which is good news for customers the length and breadth of Scotland."

Scottish Water said that by the end of its new six-year regulatory period, which commences in April, its charges will have fallen further in real terms - below the level of Consumer Price Index (CPI) inflation.

In the first three years of the agreement, from 2015-16 to 2017-18, Scottish Water household charges can increase by 1.6% per year irrespective of inflation.

Overall, in the six-year period from 2015-16 to 2020-21, Scottish Water charges must increase by no more than 1.8% below the CPI rate of inflation.