Dozens of potential terrorists directed by Islamic State are currently in Europe and have the capacity to carry out attacks, security services fear.
Britain is among the countries which are "high on the target list" for aggression from the group, the European Union's law enforcement body warned.
Europol said that - in the wake of murderous attacks in Belgium and France - extremists are likely to strike again in the near future.
All EU member states participating in the coalition against IS - including Britain - are regarded by the group as "legitimate targets".
"France remains high on the target list for IS aggression in the EU, but so too do Belgium, Germany, The Netherlands and the United Kingdom," according to a report published on Friday by Europol.
It added: "Estimates from some intelligence services indicate several dozen people directed by IS may be currently present in Europe with a capability to commit terrorist attacks, and that there are indications that IS has been preparing terrorist attacks in Europe since 2013."
Intelligence suggests that IS has assembled teams in Syria which are sent to the EU tasked with carrying out attacks, the study said.
Europol also raised the prospect of an increased rate in the return of foreign fighters if IS is defeated or severely weakened in Syria and Iraq.
Increased military pressure on IS - including a major offensive in Mosul, Iraq - has sparked concern about potential displacement of militants.
The report said: "Those who manage to enter the EU will pose a potential security risk for the Union. Given the high numbers involved, this represents a significant and long-term security challenge."
It was revealed earlier this year that around 850 people linked to the UK and regarded as a security threat are believed to have taken part in the Syrian conflict, with just under half thought to have returned to this country.
The Europol paper also said:
:: The scale and impact of "lone actor" attacks is increasing
:: A "real and imminent danger" is the possibility of elements of the Sunni Muslim Syrian refugee diaspora becoming vulnerable to radicalisation once in Europe and being specifically targeted by Islamic extremist recruiters
:: Unconfirmed information suggests German authorities were aware of around 300 recorded attempts made by jihadists to recruit refugees who were trying to enter Europe by April 2016.
:: Automatic firearms remain the weapons of choice of terrorist cells - but it is also possible that IS will consider the use of chemical or biological weapons in the EU, while car bombs could also emerge as an attack method.
:: Counter-terrorism experts are concerned that Libya could develop into a "second springboard" for IS, after Syria, for attacks in the EU and the North African region
:: Individuals and groups involved in terrorist and extremist activities use encryption to conceal their communications from law enforcement and intelligence agencies
Gilles de Kerchove, EU counter-terrorism coordinator, said: "We have to be vigilant, since the threat posed by the so-called Islamic State and returning foreign fighters is likely to persist in the coming years.
"These people are trained to use explosives and firearms and they have been indoctrinated by the jihadist ideology."
Europol director Rob Wainwright said: "The last two years have seen a number of jihadist attacks, several of which have caused mass casualties.
"The scale of this threat has been widely acknowledged in Europe, triggering an intensified cooperation between police and security services across the continent leading to an increase of arrests and plots foiled before terror attacks could be carried out."
The official threat level for international terrorism in the UK is currently severe - meaning an attack is "highly likely".
Britain's security services and counter-terrorism units have foiled at least ten attacks in the past two years.