In a series of eleven posts written by al Shabaab's Press Office on its Twitter page, the radical group threatened to inflict on Britain its worst-ever attack.

"The nightmare that surreptitiously looms on British shores is bound to eclipse the horrors of 7/7 and 21/7 combined," said one message.

The London bombings on July 7, 2005, were a series of co-ordinated suicide attacks on the city's public transport which killed 52 civilians and the four bombers.

Two weeks later, four attempted bomb attacks disrupted part of the city's public transport system.

Another message read: "Britain will pay the heftiest price for its brazen role in the war against Islam and endless brutality against innocent Muslims."

The Al-Qaeda linked group also vowed to "go to every possible length to attain the freedom of imprisoned Muslim scholars."

A 17,000-strong African Union force, fighting alongside government forces, has in recent months wrested control of a string of Shebab strongholds including the bastion of Kismayo, a strategic southern port.

Radical preacher Hamza and four other terrorism suspects were extradited to the US earlier this month after a British court rejected their last-ditch attempts to block their removal.

A legal saga that dragged on for more than a decade in the courts of Britain and Europe finally ended when two senior judges at the High Court in London dismissed the men's pleas to be allowed a stay of extradition.

The al Shabaab's messages came as three British Muslim men went on trial on Monday accused of planning a string of bombings that prosecutors said could have been deadlier than the 7/7, 2005 attacks on London.

The three men have all denied the charges.