The days of simply reading the riot act to an angry crowd are long gone.

The Home Office has been investigating the use of high-tech pain rays against mobs as an alternative to the good old water cannon, according to a report by its Scientific Development Branch due to be published next month.

The so-called active denial system (ADS) projects microwave-like radiation for distances of more than 500 yards, creating an excruciating, full-body burning sensation in anyone caught in its beam.

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The millimeter-wave rays penetrate skin to a depth of about 1/64 of an inch, but cause no permanent damage, according to Raytheon, the system's U.S.-based maker. Prototypes of the weapon, called Silent Guardian, weighed about three tons and were mounted on trucks.

"Directed energy systems such as the ADS have seen major advances over the past few years and are likely to continue to do so in the coming years," the Home Office said.

Although the report found "no options that would currently be considered", it said that might well change in the future. "We're not saying that the ADS is never going to be used. We're not going to write it off."

The Scientific Development Branch, based in Sandridge in Hertfordshire, has been looking at a portable version of the ADS being developed by Raytheon for the U.S. National Institute of Justice — which sounds suspiciously like something Judge Dredd would belong to.

The backpack-sized unit is being designed for American police. A working prototype has already been delivered.

• Click here to read the rest of the story in the Sunday Times.

• Click here for a military information page on the Active Denial System.