Intelligence officials have intercepted a credible Islamist-linked terror plot against Britain.
The threat is one of "a succession" of terror operations the country has been dealing with over the past weeks, a government official said, although the UK's terror alert status has not been raised from "severe".
The planned attack would have reportedly 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 monument in two weeks.
One UK government official said: "There have been a succession of terror operations we've been dealing with over recent weeks but one to two that have preoccupied us. Still, it hasn't been to the degree that we have raised the threat level."
Another British official would not confirm the plot was "al Qaida inspired" but said there was an "Islamist connection" and that the plots were in an early stage. No other details were given.
On Tuesday French police closed off surroundings to the Eiffel Tower, France's most visited monument. Officers pulled red-and-white police tape across a bridge leading over the Seine River, and stood guard.
Bomb experts combed through the 324m (1,063ft) tower and found nothing unusual, Paris police headquarters said. Tourists were let back inside about two hours after the structure was emptied.
German officials denied they had intercepted threats, saying there had been no change to the country's threat level.
More than 170 people were killed during a 24-hour co-ordinated attack on Mumbai by 10 gunmen in November 2008. It has been blamed on Pakistani militants. Since the 9/11 terror attacks in the US nine years ago, terror group al Qaida is believed to have moved outside Afghanistan and Pakistan to other countries such as Somalia and Yemen.