France has become the latest country to warn travellers to Britain of the heightened risk of a terrorist attack.
A message posted on the French Foreign Ministry website said the British authorities believe that "the risk of an attack is very likely".
It said visitors to Britain should exercise "extreme vigilance in public transport and busy tourist sites".
Earlier this week, the Foreign Office issued a similar warning to Britons travelling to France and Germany, while the US State Department has advised Americans visiting Europe to take additional precautions.
The warnings follow the disclosure that Western intelligence agencies had discovered that violent jihadists based in Pakistan were plotting Mumbai-style commando raids on European cities.
A report by BBC2's Newsnight said a terrorist suspect killed in a drone attack in Pakistan last month was a British man tasked with leading an al Qaida group in the UK. The programme quoted a senior Pakistani security source as saying that Abdul Jabbar was a British citizen who has a British wife and was living in Punjab, Pakistan.
According to the source, he was chosen as the leader of a new group, to be called The Islamic Army of Great Britain, at a meeting in the North Waziristan region of Pakistan three months ago.
But there was said to have been dissent at the meeting, which was attended by 300 militants and monitored by the intelligence agencies, about appointing Jabbar, who was relatively new to the group. A second meeting was called but he was killed before it could take place.
The source said the new group was set up to organise synchronised terrorist attacks in the UK, France and Germany.
It was this intelligence that led to the missile strike by a US unmanned aerial drone on September 8 which killed Jabbar and three others.