Jailed terrorists are using the internet to contact supporters

The discovery raises fears that some of the UK's most dangerous terrorists could be plotting new attacks from inside prison.

The serious security breach was revealed last week after The Mail on Sunday was alerted to 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 Britain's most feared terror suspects.

It is thought inmates are sending the illicit messages using smuggled mobile phones, many of which now allow access to the internet.

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.

The internet messages are linked to Mizanur Rahman, a 25-year-old website designer jailed last year for inciting murder after calling for British troops to be brought back from Iraq in body bags. More than 900 emails in his name appear on the Sawtul Islam website, including comments attacking homosexuals and Jews.

Non-Muslims are prohibited from accessing the forum section of the site, which is open only to people 'vouched for' by other members, who include Anjem Choudary, an acolyte of Al-Muhajiroun leader Omar Bakri Mohammed.

Explaining the "restricted access," the site says: "Do not misunderstand that there is somehow illegal speech etc being passed via this forum, that is not the case, it's more an issue of where innocent statements are taken as to mean 'encouragement to commit acts of terrorism.'"

Rahman's picture appears alongside the image of an armed man on horseback – an icon commonly used by Al Qaeda.

His personal statement reads: "Indeed our words will remain dead until we are killed for it, whereupon our words will spring to life and live amongst the hearts that are dead bringing them to life."

Last night, the Prison Service 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. Nearly 3,500 mobile phones were confiscated from UK prisoners last year.