The UK Foreign Office has updated its travel advice for Europe, warning of a "high threat" of attacks in countries including France and Germany.
Previously it had advised only of a "general threat" but an FCO spokeswoman said the safety of Britons abroad was of major concern. The US has also advised vigilance in Europe because of the threat of an unspecific al-Qaeda attack.
The UK's threat level, as set by the Home Office, remains severe.
The FCO spokeswoman said: "Like other large 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."
On its website, the FCO said : "Attacks could be indiscriminate, including in places frequented by expatriates and foreign travellers." The new level is the highest the FCO records and is based on information from a variety of sources including the Joint Terrorism Analysis Centre, security and intelligence agencies, diplomatic and media reports, local knowledge and embassy reports.
Security sources have warned of an al-Qaeda plan to send teams of gunmen to crowded places to kill civilians. They said cities in the UK, France and Germany were thought to be targets for the militants, in attacks analysts feared could be similar to the 2008 atrocities in Mumbai.
Responding to the US warning, which referred to the whole of Europe, including the UK, Home Secretary Theresa May said: "The first and most important duty of this government is the protection and security of the British people and visitors to the UK." In a statement issued on her behalf, Ms May said: "As we have consistently made clear, we face a real and serious threat from terrorism. Our threat level remains at "severe" - meaning that an attack is highly likely.
"We work closely with our international partners in countering terrorism and the US advice is consistent with our assessment.
"I would urge the public to report any suspicious activity to the police in support of the efforts of our security services to discover, track and disrupt terrorist activity."
The US state department advised Americans to take care while in tourist areas.
The department did not specify a country, issuing the updated guidance for the whole of Europe.
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