The shirts - labelled 'Made in England' - were stitched in Macedonia, formerly part of Yugoslavia, from materials manufactured in this country, before being taken to Greece to be collected.
The lorry drivers, who had left Britain 30 days earlier to collect them, were allowed home for New Year's Eve but their trailers were kept in the port.
The lorries have been held pending investigations starting tomorrow. A Customs spokesman said: 'I confirm we have detained the goods. We are investigating information that they may have been illegally imported.'
An M & S spokesman said last night: 'It is our understanding that the merchandise is only there awaiting the formal signature of a senior Customs officer prior to its release.' The company would be holding a full investigation tomorrow to establish what happened.
He added that if the shirts 'have been labelled 'Made in England', clearly an error has been made and we would not have been party to it'.
Last summer, after a row about M & S selling children's shirts made up in Serbia by a firm controlled by the Serbian leader Slobodan Milosevic, an M & S spokesman, David Sieff, said the company would not be 'sourcing' goods in Serbia again. The Department of Trade had granted it a special licence to import pounds 10m- worth of school garments from Serbia after 30 May, when United Nations sanctions cut all trade with Serbia and Montenegro in an effort to bring Mr Milosevic to the negotiating table and halt 'ethnic cleansing'.
The special licence, which was criticised by the Labour Opposition, has since expired. M & S later said it had switched production from Serbia to Macedonia, which is sanctions-free.
The lorries, operated by the Reading-based Burlington Ocean Express, had earlier been held by Italian Customs in the port of Ancona, apparently on the grounds that Macedonia is not recognised as an independent state by the European Community.Reuse content