Britain has joined the United States in warning of a heightened risk of terrorist attacks in Europe.
The US government has warned its citizens in an official travel advisory to be vigilant travelling in Europe, amid fears of an al-Qaeda commando-style attack.
The State Department advised Americans to take care while in tourist areas, but did not specify a country, instead issuing the updated guidance for the whole of Europe.
After the US alert was issued, Britain confirmed it had updated its guidance for travellers in Germany and France, warning of a "high threat" from terrorism, the BBC reports.
British Home Secretary Theresa May also said an attack in Britain is highly likely.
France said the threat of a terrorist attack was real, but that the country was not raising its alert level - currently at its second-highest.
Attack plot warning
Security sources have warned of an al-Qaeda plan to assemble teams of gunmen and send them into crowded places to kill western civilians, similar to the 2008 attacks in Mumbai.
Al-Qaeda planned to carry out co-ordinated attacks in cities in the UK, France and Germany, intelligence sources said.
After intelligence details of the plot had been leaked to the US media last week, officials said that the plan had not been stopped but that an attack was not expected to be carried out imminently.
Officials said no arrests had yet been made, and that several individuals were still under surveillance.
The suspects include British citizens of Pakistani and German citizens of Afghan origin.
Earlier, US counter-terrorism officials were reported to have said that they believe al-Qaeda leaders, including Osama bin Laden, were also involved in the plot.
Recent US drone raids in Pakistan reportedly targeted al-Qaeda militants who inspired the plans.