Paris, France - The water quality of the Seine has improved, test results showed on Thursday, three weeks ahead of the start of the Paris Olympics when the river is set to host outdoor swimming events.

Results published by the Paris mayor's office showed that E.Coli bacteria levels at an Olympics swimming spot in central Paris had fallen to within acceptable limits for four days in a row following warm and sunny weather in the French capital.

"This positive development is a consequence of the return of sunshine and warmth as well as the effects of the work done as part of the strategy to improve the quality of the Seine's waters," a statement from the mayor's office said.

The period covered June 24-July 2.

The previous week, levels of E.Coli – a bacteria indicating the presence of fecal matter – had been above the upper limits used by sports federations every day at the Alexandre III bridge location in central Paris, which is set to be the jumping-off point for the swimming events.

At one point, E.Coli levels were 10 times the upper limit of 1,000 colony-forming units per 100 milliliters (cfu/ml), with heavy rain over the previous two months leading to fears for the Olympic events.

The Seine is set to be used for the swimming leg of the triathlon on July 30-31 and August 5, as well as the open-water swimming on August 8-9.

The readings for enterococci bacteria last week – a second key measurement of water quality – also improved markedly and were within acceptable limits every day at the Alexandre III bridge.

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

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