THE HAGUE, Netherlands - Global warming is taking a heavier toll than previously thought on a gruelling 200-kilometre speedskating marathon over frozen rivers and canals linking 11 towns in northern Holland.

A study published Friday by the respected Netherlands Environmental Assessment Agency said the race is likely to be held only once every 18 years because of higher winter temperatures. Three years ago, it estimated the likelihood at once every 10 years.

Organizers insist on a minimum thickness of 15 centimetres of ice along virtually the entire route in the northern province of Friesland to ensure it is safe enough to carry thousands of skaters.

The rule means that races have always been rare. Known locally as the Elfstedentocht, it has only been held 15 times since the first official event in 1909.

"In 2005, we calculated once in 10 years, and mainly due to warm winters that came afterwards we had to change our estimate to once every 18 years so there is ... a steep decline," one of the report's authors, Arthur Petersen, said in a telephone interview. "That's purely caused by climate change."

The report, based on temperature readings from the Dutch National Meteorological Institute, was produced independently of the race organizing committee.

"The report shows that the chance of staging the 11 Cities Race is decreasing, but this does not mean that we will never have the race again," said Sybe Bruining, national secretary of the committee, in a telephone interview.

Despite German occupation of the Netherlands during the Second World War, there were four races in the 1940s -- three of them during the war.

There were two in the 1950s and then in 1963, which was recognized as the toughest ever because of a strength-sapping combination of icy temperatures, strong winds and snow.

A gap followed of 22 years until back-to-back races in 1985 and 1986. Since then, the only race was in 1997, won by brussels sprouts farmer Henk Angenent in six hours and 49 minutes -- less the half the original winner's time.

Petersen acknowledges says the Elfstedentocht is a way of focusing the public's attention on the climate change issue.

"It's an icon for climate change, but of course it is not the most important effect of climate change ... that's more related to flooding and to agriculture," he said.