The US, UK, Sweden and Japan have updated advice to citizens travelling in Europe to warn of the possibility of terror attacks there.
It comes as security sources from various Western countries warn that they are uncovering plans to attack public targets in Europe.
What is the level of the alleged threat, and how might it affect people planning trips to Europe?
What is the 'plot' these travel alerts are thought to be responding to?
Details of an alleged al-Qaeda plot first emerged in the US media, and the plot suspicions were confirmed by security sources to the BBC's Frank Gardner last week.
They told our correspondent that militants planned to carry out co-ordinated attacks in the UK, France and Germany. Small groups would seize and kill hostages, similar to the 2008 attacks in Mumbai.
Our correspondent said he believed the plot had moved from the aspirational stage to actual planning.
Information about possible targets is believed to have come from a German citizen, Ahmed Sidiqi, who was arrested in Afghanistan in July 2010.
He was one of 12 Islamists associated with a mosque in Hamburg frequented by the 9/11 attackers.
They disappeared in 2009 and were thought to have headed for the tribal areas of north west Pakistan.
Why is Pakistan so important?
A number of recent plots have originated in the area bordering Afghanistan.
The US has stepped up cross-border drone attacks and reports say the target has been al-Qaeda militants linked to the latest plot.
In one attack in which eight militants were killed, four were confirmed as Germans, prompting speculation that they may have been part of the Hamburg group that disappeared.
Which countries have issued warnings, and what have they said?
On 21 September, amid warnings from anonymous officials that terror attacks were being planned on public places, France said it was "enhancing" its vigilance.
In recent days, the UK, US and Japan have all updated their travel advice, warning of possible terror attacks in Europe and urging citizens to be cautious. Sweden has issued similar advice, and raised its threat level - though it remains lower than other European countries.
Officials in Germany and Italy say they remain vigilant, and the threat remains "high" - though no information had emerged of specific targets.
Is this the highest possible level of warning?
It differs from country to country. The US alert is a travel "advisory", less serious than a "warning", and it does not urge Americans to avoid Europe all together.
The UK Foreign Office now speaks of a "high threat" - the highest it records - rather than a "general threat" of attacks in Europe. Overall the UK's threat level remains "severe", meaning an attack is "highly likely", but it remains a level down from "critical", which warns "an attack is expected imminently".
The French alert level remains "red", the second-highest.
If people cancel their travel plans as a result of the alerts, can they expect any compensation?
It's very unlikely.
Travel industry representatives in both the US and UK point out that because there has been no explicit restriction placed against travel to Europe - the advice is to maintain vigilance - jittery travellers are unlikely to be able to receive a refund or compensation.
Some travellers may be able to postpone tickets for flights, but may have to pay a substantial penalty to do so.