High levels of E. coli were found in the River Thames by Putney Bridge where the universities race every year.

Before the event, race crews were given safety advice to avoid swallowing water splashed up from the Thames.

The Oxford University Boat Club said in a statement it could not “definitively say” what caused some of its members to get sick.

James Wallace, the chief executive of River Action UK which tested the water ahead of the race, said: “E. coli does not naturally exist in rivers and can only exist at this level if it’s from sewage.”

The number of hours in which sewage was dumped into the River Thames have more than quadrupled in the last year, figures analysed by London City Hall in January show.

Thames Water blamed the rise on higher than average rainfall.

Joan Fennelly, a fellow Henley Mermaid, said she was “absolutely disgusted but also not surprised” about the Oxford rowers becoming unwell.

She told BBC London she had been made “really, really sick” as a result of swimming in the River Thames over recent years.

Ms Fennelly added with London facing the “cumulative effect of all of the sewage discharges” it is the “worst place that you could possibly go into the water”.