From the late 1980s onwards, Britain's secret eavesdropping agency GCHQ intercepted Irish international communications as they passed through England.
This would have given Britain access to secret conversations involving members of the Irish government in the years leading up to the Good Friday Agreement.
To listen in, GCHQ built a 150-ft tower at a quiet location in Cheshire at a cost of pounds 20m.
Intercepted calls were processed in the tower and sent for transcript and analysis at GCHQ's headquarters in Cheltenham.
The Ministry of Defence is now selling the strange windowless tower. Bids are open to midday today.
Details of the tower's powerful and sophisticated electronic facilities were revealed last night in a Channel 4 News report.
Programme makers were shown round the tower by officials from the Ministry of Defence's Defence Estate Disposal organisation.
According to senior intelligence sources, although the primary justification of the interception network was to obtain information about terrorism, the station also obtained political and economic intelligence from the Irish government and the business community.
It, and others like it, targeted commercial and diplomatic communications from the Irish Republic.
And it also homed in on the personal communications of prominent Irish citizens using voiceprint recognition systems and lists of target telephone numbers.
The tower was closed early in 1998 after a new Irish communications systems replaced the radio links it was built to intercept.
Intelligence sources say it has been superseded by an even more powerful network of Ministry of Defence owned optical fibre cables running in a ring around England.
Another GCHQ system is said by intelligence sources to intercept most of Britain's Internet and e-mail communications as they pass through a key Internet site in London's Docklands.
Listening in, page 4