The charity River Action submitted a Freedom of Information request to the Environment Agency, which revealed that in 2021 raw sewage was discharged into the two principal river catchments of the New Forest - the Lymington and Beaulieu Rivers - on more than 250 occasions and a total period of just under 1,900 hours.
The most frequent sewage discharges over this period in the National Park have occurred at Brockenhurst Wastewater Treatment Works on the Lymington River and Lyndhurst Wastewater Treatment Works on the Beaulieu River, accounting for over 50 per cent of total discharges.
The chairman of River Action, Charles Watson, has written a letter to the CEO of Southern Water expressing his "deep concern" over the uncovered data.
He said it was "simply scandalous" that New Forest wetlands are being used in this way considering their importance as a habitat for wildlife and biodiversity.
"In the case of the discharges from Lyndhurst Wastewater Treatment plant, the discharges go straight into the vulnerable headwaters of the Beaulieu River and its associated flood plain – all of which is a highly protected Site of Special Scientific Interest," the letter reads.
"This desecration of this iconic nation park by Southern Water must end with immediate effect," it concluded.
River Action said more than half of the New Forest – voted the number one national park in Europe in TripAdvisor awards in 2022 – was designated for its importance for nature, with its wetlands particularly important for wildlife.
Professor Russell Wynn, director of local wildlife group Wild New Forest said: "The rivers and streams of the New Forest should be some of the cleanest in the country, but they are regularly being flushed with a noxious mix of pollutants expelled from wastewater treatment works.
"Inadequate monitoring of fauna and flora in and adjacent to the affected rivers means we simply don't know what impact these regular discharges are having on protected habitats and species, and the extent of the long-term damage being done.
"We alerted the local community and our elected representatives to this issue two years ago, but we are yet to see a firm commitment by Southern Water to prioritise investment in this highly protected landscape."
In a statement, Southern Water said: "We are one of the key stewards of river water quality and are investing record amounts to protect and enhance the environment.
"In particular our Storm Overflow Task Force is leading the industry in our pioneering approach to reducing releases during heavy rain.
"Nature-based solutions and working with partners including NGOs (non-governmental organisations), landowners, councils and businesses is the key to reducing the amount of water getting into our system and slowing the flow so our systems can cope."
