The Rhine has been hit by its longest period of low water in 40 years, Dutch officials said Wednesday, raising inland shipping costs and fears of collisions on one of Europe's busiest rivers.

"The Netherlands is currently hit by the lowest water levels in the Rhine since 1976," said the Rijkswaterstaat, the department tasked with dealing with transport infrastructure.

On Monday "the 1976 low water level record of 120 days was broken and it's expected that the situation will continue," it explained in a statement.

As a result, the river's draft has fallen by at least a third, meaning more but smaller ships are needed to transport the cargo.

That in turn increases the risk of collision as the mighty waterway -- an artery serving the Netherlands, Germany, France and Switzerland -- narrows in width due to falling water levels.

"The situation in Germany is particularly worrisome," said Marja de Vet, editor at the Dutch specialist shipping magazine Schuttevaer.

"As from this week, large ships are unable to get through a passage in the river," she said, referring to Germany's Rhine Gorge.

The Gorge is notorious for its strong currents including at the Lorelei, a huge rock on a scenic but particularly dangerous bend in the river.

"At this moment the water level is so low that only a limited number of our ships can use the river," added an official at a Rotterdam inland shipping company.

"With a narrowing in the waterway, there is indeed more risk of collision," he said, asking not to be named.

The Rhine is one of the world's most frequented inland waterways and some 600 ships cross the Dutch-German border on a daily basis, says the Strasbourg-based Central Commission for the Navigation of the Rhine.

Some 310 million tonnes of freight are transported annually on the Rhine's Dutch section alone, the five-nation CCNR said on its website.

In total, around 6,900 ships ply their trade along more than 1,000 kilometres (600 miles) of one of Europe's best-known rivers, which has served as a waterway for hundreds of years.

Low rainfall in the Rhine's catchment area and possible climate change factors as well as winter conditions with snow could well mean that the situation on the Rhine could stay the same or worsen at least until spring next year, shipping experts say.