The 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."
'Indiscriminate attacks'
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.
In the UK the highest level that can be reached is critical - when an attack is expected imminently. The current threat level indicates an attack is "highly likely".
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."
Foreign Secretary William Hague said the updated travel guidance from the US "reinforces the need for vigilance".
"We can't comment on the specifics but the need for vigilance is very strong," he said.
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.
A White House spokesman said: "From the day we became aware of this latest plot, the president made clear we need to do everything possible to disrupt this plot and protect the American people. Whether the State Department issues a travel alert is the decision of the State Department but this alert is responsive to the President's direction that we spare no effort."
