More than 10,000 child refugees have disappeared since arriving in Europe, with many likely to have fallen into the hands of traffickers, the EU's criminal intelligence agency has said.
Europol said that thousands of vulnerable young people had gone missing after registering with authorities.
It is believed that 5,000 children had disappeared in Italy, while a further 1,000 were unaccounted for in Sweden.
Europol's chief of staff, Brian Donald, said that there now existed a sophisticated pan-European "criminal infrastructure" that was targeting refugees.
"It’s not unreasonable to say that we’re looking at 10,000-plus children. Not all of them will be criminally exploited; some might have been passed on to family members.
"We just don’t know where they are, what they’re doing or whom they are with," he told the Observer.
The British government announced last week that it would work with the United Nations refugee agency - the UNHCR - to identify "exceptional cases" of unaccompanied children in war zones.
The move follows calls from charities led by Save the Children for Britain to admit at least 3,000 young people who have reached Europe, warning that many simply "disappear" once here and fall into the hands of people traffickers.
Europol, which has a 900-strong force of intelligence analysts and police liaison officers, believes 27% of the million arrivals in Europe last year were minors.
In October, officials in Trellebory, Sweden, said that about 1,000 refugee children who had arrived at the port town alone in the previous month had gone missing.
Donald said that Europol had received evidence that some refugee children who had arrived alone had been sexually exploited.
He said: "An entire [criminal] infrastructure has developed over the past 18 months around exploiting the migrant flow. There are prisons in Germany and Hungary where the vast majority of people arrested and placed there are in relation to criminal activity surrounding the migrant crisis."
He urged the public to remain vigilant and "alert" to the situation, saying that the children who had gone missing "would be hiding in plain sight".
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