Britain's foreign ministry revised its travel information for Germany and France, saying the threat of terrorism was "high" in both countries.
"We can confirm that the travel advice for France and Germany has been updated," a foreign office spokeswoman told AFP.
Earlier Sunday, the US State Department released an official alert for Americans travelling to Europe, reminding US citizens to be aware of the chance that attackers could target "public transportation and other tourist infrastructure."
"Current information suggests that al-Qaida and affiliated organisations continue to plan terrorist attacks," the alert stated. "European governments have taken action to guard against a terrorist attack and some have spoken publicly about the heightened threat conditions."
The State Department alert did not list any specific warnings for individual countries in Europe.
UK Home Secretary Theresa May called on British nationals to report any suspicious activities to authorities.
Citing intelligence officials and people familiar with the matter, National Public Radio in the United States reported Friday that al-Qaida boss Osama bin Laden sent a directive to the terrorist network's partners months ago, ordering a Mumbai-style attack on at least three European countries – Germany, Britain and France.
More than 160 people were killed in the 2008 shooting and bombing attacks in India's largest city.
Some officials worried that members of the commando-style teams could be travelling to the West using European passports, thus complicating any effort to find and stop them.
Despite media reports, the German Interior Ministry announced Wednesday that it would not raise the threat level for the country.
Der Spiegel magazine reported that the latest terror plot was uncovered during interrogations of a German-Afghan detainee being held at the Bagram US army base in Afghanistan.