Tourists travelling to France and Germany have been warned that the threat of terrorism has risen to its highest level as fears grow of an imminent Mumbai-style attack in Europe.
In a series of official warnings, visitors to Britain were also alerted to the heightened threat level by the US State Department, which said transport hubs or tourist sites could be targets.
The move came amid continued concerns that al-Qa'ida was plotting to send gunmen to the streets of major cities for commando-style raids.
Plans for the attacks were disrupted by drone aircraft bombings in Pakistan's tribal areas in recent weeks but the warnings reflect a concern that the terrorists may have an extensive support network in Britain and elsewhere.
Theresa May, Britain's Home Secretary, said last night that there remained a "severe" threat level, "meaning that an attack is highly likely".
Security around the royal family had been increased, sources confirmed.
The British Foreign Office, in its official travel advice, said there was a "high threat" of attacks in countries including France and Germany, rather than the "general threat" previously identified.
"Attacks could be indiscriminate, including in places frequented by expatriates and foreign travellers," it added.
US counter-terrorism officials have said the threat was "credible, but not specific" and it is understood that MI5 has been heavily involved in trying to trace extremists in Britain who may have been in touch with terrorists in Pakistan.
It is believed the plans included having terrorists open fire on the streets, probably of capital cities, before heading into landmark buildings, as in the Mumbai attacks in 2008.
The US alert issued yesterday noted in particular "the potential for terrorists to attack public transportation systems and other tourist infrastructure" and advised Americans to "take every precaution, to be aware of their surroundings and to adopt appropriate safety measures to protect themselves when travelling".
On Friday, Sweden raised its threat alert to the highest level ever. In France, the Eiffel Tower has been evacuated twice in two weeks amid fears that the country has been targeted by terrorists from North Africa. (©Daily Telegraph, London)