Levels of common antibiotics which are three times the "safe" concentration have been found in the River Thames, according to a new study.
The river was found to be contaminated with five different drugs, including Ciprofloxacin - used to treat skin and urinary infections - which was found to be at triple the concentration which is considered safe.
The University of York study tested for 14 common antibiotics in 72 countries, finding the drugs at two-thirds of the sites.
Researchers found a maximum total antibiotic concentration of 233 nanograms per litre in the Thames, but in Bangladesh the concentration was 170 times higher.
The Danube was found to be Europe’s most polluted river, with eight per cent of the continent’s sites above safe limits.
More than a third - 35 per cent - of river sites tested in Africa exceeded safe limits, followed by 23 per cent in Asia and 18 per cent in South America.
Dr John Wilkinson, one of the authors of the report, told The Times: "We're really beginning to understand that there are concentrations of antibiotics in the environment above which bacteria may select for resistance to antimicrobials.
“The ultimate worry here is that these pathogens may become resistant to the medication we use to kill their infection in humans."
Last month, the United Nations said that antibiotic resistance could cause 10 million deaths a year by 2050.
Alistair Boxall, an environmental scientist who co-led the study, told The Guardian: “It’s quite scary and depressing. We could have large parts of the environment that have got antibiotics at levels high enough to affect resistance.”
