The river Seine is still failing water quality tests one month before the Paris Olympics when it is scheduled to host the open-water swimming competition and the swimming leg of the triathlon, results showed Friday.

The latest tests, completed last week and released by the Paris mayor's office, showed levels of the E.Coli bacteria – an indicator of faecal matter – are far above the upper limits imposed by sports federations.

"Water quality remains degraded because of unfavourable hydrological conditions, little sunshine, below-average seasonal temperatures and upstream pollution," the mayor's office said in a statement.

French authorities have spent 1.4 billion euros (US$1.5 billion) in the last decade trying to clean up the river by improving the Paris sewerage system, as well as building new water treatment and storage facilities.

But major storms still overwhelm the capital's waste water network, some of which dates back to the 19th century, leading to discharges of untreated sewage directly into the river.

Organisers insist that some dry weather and sunshine in July should be sufficient to make the Seine fit for the outdoor swimming events. (AFP)