The number of bathing water in England deemed to be poor and unsuitable for swimming has risen from 18 to 37.
Of the 450 sites regularly tested for bacteria associated with sewage discharges this summer, 92% of them reached the minimum standard for bathing, down slightly from 96% last year.
The Environment Agency – which carried out the tests – says the figures are partly due to monitoring 27 new sites this year, 18 of which were rated poor.
The government described the latest figures as “unacceptable”.
Water Minister Emma Hardy blamed England’s water companies and said she stressed the need for tougher regulation.
Alan Lovell, head of the Environment Agency, said the findings came against a backdrop of growing demand for bathing facilities across the country.
“Overall bathing water quality has improved in recent decades due to targeted investment and stronger regulation, but today’s results show that much work remains to be done, particularly in our To bring domestic bathing water up to standard,” he said.
The data show a marked difference between the quality of beach and inland bathing facilities.
95% of coastal waters met the minimum standards this year, compared to only 53% of rivers and lakes.
The Environment Agency says this is because salt water can act as a natural disinfectant and the ocean naturally disperses pollutants quickly.
The quality of England’s bathing water has come under increasing criticism in recent years, as more data has become available.
RiverAction CEO James Wallace called the latest findings “an international embarrassment”.
“The government’s own data shows that swimming in our inland bathing areas poses serious health risks, highlighting the failure of regulators to protect waterways from pollution,” he said.
Specific bathing areas are tested 10 to 20 times a year for bacteria associated with sewage discharge.
At the end of each season the Environment Agency gives them a rating based on their results over the previous four years.
Excellent, good or sufficient means they meet minimum standards – while poor means they don’t.
A spokesman for Water UK, which represents the water industry, said the quality of English bathing water had improved since the 1990s but water companies had agreed to reduce sewage discharges. More work needs to be done.
The companies are currently awaiting approval from the regulator of Watt to invest billions of pounds in infrastructure, which will be paid for through higher consumer bills.