Spanish National Police said the investigation — named operation Kolso — was launched after 56 Ukrainian migrants were detained at 12 Spanish airports for carrying fake documents, which cost them anything between €300 and €1,500.

The majority of them were booked on flights to Dublin.

Spanish police said that since Russian forces had annexed Crimea from Ukraine, many Ukrainians had fled the country in the hope of making it to Britain.

After seizing documents and questioning the migrants they raided a house in Barcelona where they arrested a man and confiscated more than 100 identity cards and forged Polish passports.

By now Spanish police had broadened their inquiries and with the help of Europol they enlisted the help of police and border forces in Belgium, France, Poland and Britain.

A further 30 Ukrainians were detained with false documentation at airports in Belgium, 21 were held in France and three each in airports in Portugal and the Netherlands.

The arrests in Spain were made at airports in Barcelona, Murcia, Santander, Tarragona, Madrid, Palma de Mallorca, Ibiza, Tenerife, Gerona, Bilbao, Malaga and Alicante.

A Romanian man was then arrested and is claimed to be one of the kingpins of the organisation who vetted the Ukrainians before they were given false documentation.

In several cases the falsified documents were extremely well produced.

A Europol source said that the cheaper ones were issued with lookalike photographs while the more expensive ones featured the picture of the person who was actually travelling.

The Spanish say that risk analysis carried out by the European border agency, Frontex, clearly indicated that in recent years a noticeable number of Ukrainians travelling with false documents passed through Dublin Airport.

The Ukrainians were given temporary accommodation in Ireland until gang members put them on ferries to Britain.