Drawn on a wooden frame to the execution site, hanged until almost dead, but still conscious, castrated, disembowelled and finally beheaded, bodies quartered and displayed with the head in public to deter others -- Britain historically dealt mercilessly with traitors.

The centuries-old charge of betraying the monarch and his or her government no longer carries the death penalty, but it is still one of the gravest crimes on the statute books and condemns those found guilty to life imprisonment.

Britain's top legal authority, Lord Charles Falconer, has played down speculation that radical Islamic clerics could be tried for treason under a set of anti-terror measures that are being compiled following the London bombings.

Experts, however, noted that extremists plotting attacks against Britain and those who support or help them would likely fall under the legislation, which was first embodied in the Treason Act 1351.

At the same time, they argued that a raft of other, more modern, laws to counter terrorism would be more suitable in dealing with the problem.

"I suppose the attraction of prosecuting for treason as against an offence under the explosives act or all the other pieces of legislation is that it symbolically encapsulates the fact that they have made an attack on the state," said John Spencer, a professor of law at the University of Cambridge.