The safety of the Royal Family and top politicians is at risk because 'classified' security details are being published on the internet.
Scanning enthusiast Paul Wey has admitted to the BBC that he is intercepting Special Branch and other security communications and publishing the details on internet news groups.
Wey, from Hertford, told the Today programme that he had a list of frequencies used by security agencies for big events or organisations in London.
He admitted that what he was doing was illegal, but denied that he was giving information to terror groups.
"[Terrorists] would be aware of these things whether I published them or not," he told the BBC.
Wey blamed the fact that Special Branch's radio equipment was so out of date that it could easily be scanned.
A BBC defence source described Wey and his website as a "menace" and "a severe danger to the public and to national security".
"In the wrong hands [this] could be used to the detriment of Crown and government," he said. "It could be used by terrorists to perpetrate fairly serious atrocities."
The source suggested that Wey's newsgroup may have been used by Millwall supporters to counter police operations against them during riots at the south London football club on 2 May.
But Wey denied this, maintaining that he was "not having people like that on the group".
But the radio programme claimed to have evidence that Wey provided supporters with radio frequencies.
The options for action which the UK authorities could take against Wey are limited, since offenders have to be caught using a scanner to be charged.
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