A large number of these illegal immigrants are staying in and around the capital, intelligence sources said.

Among the illegal foreigners around 25,000 who have entered the country through the Zia International Airport are enlisted in the official records. Others have entered Bangladesh using the 13 land ports, according to the Department of Immigration and Passports (DIP).

"After the serial bomb blasts across the country on August 17, we are now considering these people as threats to our national security," said an intelligence official.

Most illegal immigrants have been living in the capital and in the port city of Chittagong for more than three years using false addresses after their visas had expired.

"Many had written the names of Pan Pacific Sonargaon Hotel and Hotel Sheraton as their addresses while in Bangladesh but when we went to those hotels, we never found any trace of them," said an official of the Special Branch (SB).

Sources said as the land ports lack data recording systems, that is why the government remains in the dark about the whereabouts of these illegal immigrants. Intelligence agencies suspect that some of these people might be involved in subversive activities.

The intelligence agencies have recently come up with a 624-page index of listed illegal immigrants, which will be submitted to the Ministry of Home Affairs soon. Sources said the list mentions some 19,000 Indians, 2800 Pakistanis and 2682 Chinese nationals. There are people from other countries also. The illegal Pakistani immigrants have been staying in Bangladesh for the longest period, the index observed.

Among the listed illegal immigrants, many are working in the Information Technology (IT) sector, English medium schools, private universities and different corporate houses.

"They live and work here without any legal work permit. The government is being deprived of revenues, as they do not pay taxes. Their free movement is also a huge security concern," said a senior official of the Special Branch. Officials of the immigration and passport department believe that many illegal immigrants are managing to stay in the country without extending their visas as they have developed good understanding with a section of dishonest officials of the visa cell of the department.