The level of threat has been raised from 'general' to 'high' because security services in Europe have uncovered an Al-Qaeda plot to attack civilians in crowded places.
An FCO spokeswoman told the BBC today, "Like other European countries, [France and Germany] have a high threat of terrorism, which is reflected in our updated travel advice."
"We therefore attach great importance to providing information about personal safety and security overseas to enable people to make informed decisions about travel," she said.
The U.S. State Department has also increased its guidelines, advising caution throughout the whole of Europe.
The alert level for the U.K. itself is unchanged, at 'severe,' which means that an attack is highly likely. It's little comfort to residents here and visitors to Britain that there is a level above this -- 'critical' -- when an attack is perceived to be imminent.
Although the main Mediterranean cruise season is at its tail end, France and Germany are still busy with autumn river cruises, which pass through the major cities, and there are still plenty of ships calling at British ports.
Stay tuned and we'll report tomorrow on the reaction of the cruise lines.
--by Sue Bryant, Cruise Critic Contributing Editor