More than 1,300 Britons are being struck down by bladder cancer each year because of disinfectants found in tap water, according to new research.
Only Spain has a worse record across the whole of the European Union, scientists have warned.
Almost one in twenty cases can be traced back to trihalomethanes (THMs) that are created as a by-product of chlorination.
The tumours of more than 6,500 people in 26 European countries diagnosed with the disease annually are fuelled by the chemicals.
The maximum reported concentrations exceeded the limit of 100 micrograms per litre in nine countries - including the UK.
Spain and the UK had the largest number of cases of bladder cancer blamed on THMs - 1,482 and 1,356, respectively.
Co-author Dr Manolis Kogevinas said: "Over the past 20 years, major efforts have been made to reduce trihalomethanes levels in several countries of the European Union, including Spain.
"However, the current levels in certain countries could still lead to considerable bladder cancer burden, which could be prevented by optimising water treatment, disinfection and distribution practices and other measures."
In the first study of its kind the Barcelona Institute for Global health team analysed levels in the tap water of 26 EU countries - all apart from Bulgaria and Romania.
Co-ordinator Dr Cristina Villanueva said: "The biggest challenge was collecting representative data on national trihalomethanes levels.
"We hope these data will become more readily available in the future."
Earlier research has found a link between long-term exposure to the chemicals through ingestion, inhalation or skin absorption and bladder cancer.
The latest findings published in Environmental Health Perspectives are based on questionnaires sent to bodies responsible for municipal water quality.
They included information on the concentration of THMs spanning 2005 to 2018 at the tap, in the distribution network and at water treatment plants.
Complementary data was also obtained online from reports, scientific literature and other sources.
Considerable differences between countries were identified. The average level of THMs in drinking water in all countries was well below the permissible limit - 11.7 micrograms per litre.
But the highest concentrations exceeded the 100 micrograms per litre limit in Cyprus, Estonia, Hungary, Ireland, Italy, Poland, Portugal, Spain - and the UK.
Lead author Dr Iro Evlampidou described the study as being "of particular interest to countries with high average levels of trihalomethanes concentrations recorded in tap water."
The number of attributable bladder cancer cases was estimated through a statistical calculation using international information on incidence rates for each country.
In total, the researchers estimated 6,561 bladder cancer cases per year in the EU are caused by exposure to THMs.
Considerable differences were found between countries. The alarming rates in Spain and the UK were partly due to high incidence of the disease and their large population.
Those with the highest percentage of bladder cancer cases attributable to THM exposure were Cyprus (23%), Malta (17%), Ireland (17%), Spain (11%) and Greece (10%).
Efforts to reduce the problem should focus on countries with the highest average levels, said the researchers.
If the 13 countries with worst records were to reduce their THMs to the EU average, an estimated 2,868 annual attributable bladder cancer cases (44%) could be avoided.
