The US government is to issue a travel alert, warning its citizens to be vigilant while travelling in Europe because of the threat of an al-Qaeda commando-style attack.
US and UK officials have confirmed that updated guidance will be issued because of the current terror threat.
A UK official said the travel advisory would not be country-specific.
It would also not go as far as advising against travel to Europe, the official said.
The advisory will be issued in response to intelligence on an al-Qaeda plan to assemble teams of gunmen and send them into crowded places to kill western civilians, similar to the 2008 attacks in Mumbai.
Al-Qaeda planned to carry out co-ordinated attacks in cities in the UK, France and Germany, intelligence sources said.
After intelligence details of the plot had been leaked to the US media last week, officials said that the plan had not been stopped but that an attack was not expected to be carried out imminently.
Officials said no arrests had yet been made, and that several individuals were still under surveillance.
The suspects include British citizens of Pakistani and German citizens of Afghan origin.
US officials said that the travel alert would be issued on Sunday.
Such a warning could have negative consequences for European tourism if travellers fear that there is a risk of terror attacks and cancel their journeys.
However, an alert is less serious than a travel warning.
Earlier US counter-terrorism officials were reported to have said that they believe al-Qaeda leaders, including Osama Bin Laden, were also involved in the plot.
Recent US drone raids in Pakistan reportedly targeted al-Qaeda militants who inspired the plans.
The US has carried out at least 25 drone strikes so far this month in Pakistan's tribal areas - the highest monthly total for the past six years, US media reported.
US officials have been pushing Pakistan to increase their search for the militants, who are believed to be hiding in a mountainous border region in the country.