The serious security breach was revealed after the newspaper repoprted the messages on a ''private'' website linked to militant Islamic movement Al-Muhajiroun, which has been banned under anti-terror laws. The threatening emails were apparently sent from inside Belmarsh high-security prison in South-East London, which houses some of most feared terror suspects.
It is suspected that inmates are sending the illicit messages using smuggled mobile phones, many of which now allow access to the internet.
The internet messages are linked to 25-year-old website designer Mizanur Rahman jailed last year for inciting murder after calling for British troops to be brought back from Iraq in body bags.
The jail's current prisoners include Abu Doha, who is accused of plotting to bomb Los Angeles Airport, and Dhiren Barot, who was jailed for life for planning terror attacks, including one using a radioactive ''dirty'' bomb.
Last night, the Prison Service, however, said inmates did not have internet access ''except under exceptional and controlled circumstances, and cannot personally contribute to websites.'' Sources said an investigation had been launched into the matter.