British and American intelligence sources say there is a high risk of a terror attack in Europe.
Britain followed the US State Department in issuing a warning that Al-Qaeda was planning to strike somewhere in Europe.
The Foreign Office said there was an increased threat in France and Germany and advised its nationals to be vigilant.
French Interior Minister Brice Hortefeux said the threat of a terrorist attack was real, but that the country would not raise its alert level – currently at its second-highest.
“We don’t want to worry for our fellow citizens because our security service are taking all measures necessary. At the same time we must not be in denial, there is actually a stream of consistent intelligence, which has been confirmed by the Americans,” Hortefeux said.
Both Britain and the US say their nationals should be careful when visiting tourist hotspots across the continent.
No arrests have been made, but several individuals are being watched by the security services.
The suspects include British citizens of Pakistani origin and German citizens of Afghan origin.