Many Celtic fans are expected to travel to Italy for Wednesday's second leg of the last-16 tie after obtaining seats among the Italian supporters through travel agents.

Thousands could now be turned away and have been urged not to attend.

Milan officials and Strathclyde Police insist the crackdown on stadium security in Italy means those who missed out on the club's allocation of 4,500 tickets for a designated away section will be refused entry.

AC Milan organising director Umberto Gandini said: "It is absolutely crucial that Celtic fans only obtain tickets for the areas of the stadium allocated to visiting support and do not attempt to enter parts of the San Siro occupied by home fans.

"Celtic supporters will be refused entry to areas of the stadium which have been designated for the home support.

"The Milan police have made it clear that spectators will be asked to produce photo identity to gain entry to the stadium.

"This will be in place and is intended to ensure that the agreed segregation of fans is strictly followed.

"We look forward to welcoming Celtic fans in Milan but would ask all supporters to adhere to the advice given in the interests of safety."

San Siro officials have overcome plenty of red tape to get the match on in Milan's home stadium.

New turnstiles have been put in place, and further modifications carried out after police chiefs and the Italian government demanded all stadiums in Italy correspond to new minimum standards.

The move followed the death of a police officer, Filippo Raciti, after violent clashes at the Serie A derby between Catania and Palermo on February 2.

Milan dare not risk any trouble breaking out, as it could force them to play future matches behind closed doors.

Kenny Scott, Chief Superintendent of Strathclyde Police, said: "Only supporters in possession of tickets for the official Celtic section of the stadium are guaranteed entry.

"Following on from a meeting involving AC Milan, Celtic Football Club and Strathclyde Police in Italy last week it is clear that the intention of the Italian authorities is to carry out identity checks on all supporters attempting to enter the stadium.

"A two-tier security check at the perimeter of the stadium will facilitate this. It is a condition of entry to Italian stadia that evidence of identity must be produced on request.

"In the case of the match next week such evidence of identity may require proof of nationality.

"Despite information in the press from travel agents who have supplied tickets for other areas of the stadium than the official Celtic area it is the clear understanding of Strathclyde Police that the above checks will take place and that entry will be refused as stated.

"Whilst there is no doubt that such statements have been made in the past and have not been followed through, there is clearly a strong desire on the part of Italian authorities to improve access procedures in light of events in Sicily at the Catania v Palermo match and the subsequent introduction of new football-related legislation.

"The advice of Strathclyde Police to persons intending to travel to the match with tickets for other sections of the San Siro stadium than the official Celtic area is that they should not attempt to attend the match."