Britain followed the US in upgrading its travel advice for Europe today following a series of terror alerts.
The Foreign Office warned that there was a "high threat" of attacks in countries including France and Germany, rather than the "general threat" previously identified.
The move came shortly after the State Department issued guidance urging Americans to be vigilant when visiting Europe, highlighting the "potential for terrorists to attack public transportation systems and other tourist infrastructure".
The Foreign Office advice now states: "Like other large European countries, the French/German authorities continue to consider that there is a high threat of terrorism.
"Attacks could be indiscriminate, including in places frequented by expatriates and foreign travellers."
The warnings have been heightened after intelligence officials in Britain intercepted a credible al Qaida-linked terror plot last week.
The planned attack would reportedly have been similar to the deadly commando-style raids in Mumbai, India, two years ago, with other European cities, in France and Germany, also targeted.
On Tuesday night the Eiffel Tower in Paris was evacuated following a bomb threat called in from a telephone booth. It was the second such alert at the tower in two weeks. A search by bomb experts found nothing unusual, and it was reopened within hours.
On Friday, Sweden announced it has raised its threat alert to the highest level ever because of an increased threat of terror attacks.
There has been speculation that Osama bin Laden could be masterminding the latest plots personally.
Home Secretary Theresa May urged the British public to report any suspicious activity as police and security services try to disrupt terrorist activity.
"The first and most important duty of this government is the protection and security of the British people and visitors to the UK," Mrs 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.
"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."
Mrs May said the UK was working closely with international partners including the US on counter-terrorism, and its new guidance was "consistent with our assessment".
The State Department stopped short of recommending that citizens stay away from high-profile sites in Europe - as was thought to have been under consideration before.
"Current information suggests that al Qaida and affiliated organisations continue to plan terrorist attacks," the advice said. "European governments have taken action to guard against a terrorist attack and some have spoken publicly about the heightened threat conditions."
It noted in particular "the potential for terrorists to attack public transportation systems and other tourist infrastructure".
"US citizens should take every precaution to be aware of their surroundings and to adopt appropriate safety measures to protect themselves when travelling," the department said.