The gang allegedly supplied fake Spanish passports to Iranian nationals so they could fly into the UK.

More than 100 people were arrested across Europe, including the suspected ringleader, who was apprehended at London's Heathrow airport.

The operation involved Spain's federal police, London's Metropolitan Police, and European crime agency Europol.

Europol said the group, operating from Malaga in the south of Spain, charged the migrants about €25,000 (£22,500) each for fake Spanish passports, travel documents, transfers and accommodation.

Spanish police said the criminal organisation was 15 people-strong, all of whom had been detained. All were arrested in Malaga, police said, except for the suspected ringleader at Heathrow - who was trying to flee to Brazil.

He was detained on a European arrest warrant.

Forty-two of the people detained were accused of selling their genuine Spanish documents to the criminal group - for anywhere between €300 and €5,000.

Dozens of Iranian nationals were also arrested at European airports, after being found carrying the forged passports.

During the raid in Malaga, police found records of seven Iranian migrants, including a five-year-old child.

They also seized dozens of real passports, computers, printers, more than 400 blank ID cards - as well as guns, cash, and "a high-end vehicle".

The year-long investigation began when several Iranians were found in Hamburg, Germany, with a mix of fake and genuine passports, and tickets from a travel agency based in Malaga. That discovery led police to the large-scale smuggling operation.

"The criminal group was perfectly structured," Europol said in a statement.

"Each member had a defined role, ranging from recruiting the irregular migrants in their country of origin, to facilitating the transfers, hosting them in safe houses in Spain, and supplying the travel documents."

Spain's Policia Nacional said: "With the dismantling of this criminal organization, the illicit trafficking of Iranian citizens to the EU and the United Kingdom has been considerably reduced."