Dry weather in Germany and Switzerland has led to a steady fall in Rhine levels since late October, with freight vessels sailing only 20-50% full, according to a Reuters report.

Vessels lengths are also being restricted.

Avalon Waterways has been forced to transfer passengers on to vessels with shallower drafts so two cruises can continue uninterrupted, according to a report today on website Cruisecritic.com

The company is quoted as saying that at this stage it intends operating all of its Christmas sailings.

But it’s looking at various options, including lightening loads and using coaches to take passengers beyond sections which vessels can’t navigate.

Viking told the website that it is looking at similar contingency moves.

It says passengers can cancel and obtain a credit for another sailing, but so far the "vast majority" are sticking to original plans.