The order for two extra ships takes the number of new river yachts up to six - the previously announced four ships – Crystal Debussy, Crystal Ravel, Crystal Mahler and Crystal Bach – are due to be unveiled in summer 2017.

All of these six ships will be operating on European rivers by 2019.

Also included is the first purpose-built Polar Class yacht, Crystal Endeavor, which will sail to the Arctic and Antarctic regions when it launches in August 2018.

The final vessel is the first of the new Crystal Exclusive Class ships which will cater for up to 1,000 passengers and is set for delivery in autumn 2019.

All eight new ships are to be built by the Lloyd Werft Group of shipyards in Germany.

Crystal chief executive Edie Rodriguez said: “The new Exclusive Class ships, yachts and river yachts planned for the coming years provide travellers with an exhilarating glimpse into the future of luxury travel of all kinds, from rugged expedition adventures to indulgently relaxing river cruises.”