Child refugees are falling into the hand of organised trafficking syndicates, warns the European Union's criminal intelligence agency. Brian Donald, Europol's chief of staff said that 5,000 children had disappeared in Italy, their whereabouts unknown, while a further 1,000 were missing in Sweden.

"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," Donald told the Observer. "We just don't know where they are, what they're doing or whom they are with."

This is the first undertaking by law enforcement agencies to find out what is happening to thousands of minors coming into Europe from war-torn regions such as Syria.

According to Save the Children, around 26,000 unaccompanied children entered Europe last year. Europol believes 27% of the million arrivals in Europe last year were minors. "These children need and deserve our help and protection. Left to fend for themselves, lone children are extremely vulnerable along the refugee route. Children report having been beaten, extorted, and sexually abused on their journey," Save the Children CEO Justin Forsyth said.

In the UK the number of children who disappear soon after arriving as asylum seekers has doubled over the past year, raising fears that they are also being targeted by criminal gangs. Britain is accepting an unspecified number of unaccompanied child refugees from Syria and other conflict zones.

Campaigners had called for the UK to take 3,000 children from Europe but Prime Minister David Cameron said that doing so might spur more refugees to risk "lethal" journeys.

Europol have received evidence that unaccompanied child refugees in Europe had been sexually exploited, Donald said. In Germany and Hungary, large numbers of criminals had been caught exploiting migrants.

"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."