GCHQ complained earlier this year that technology which allows Internet signals to be transmitted over power cables makes it harder to do its job by interfering with radio communications.

A letter has come to light in which the security communications unit said that by interfering with various radio frequencies, powerline transmission (PLT) technology "is likely to cause a detrimental affect [sic] to part of the core business of this Department".

The letter does not specify which part of its business will be affected, but it is believed to refer to the interception of radio communications for national security purposes.

The letter remains available on the website of campaigning organisation Ban PLT.

Powerline transmission uses electrical wiring to route an Internet connection around a building, allowing multiple devices to share the same network connection.

Critics say that because powerlines were not designed to transmit data, they are prone to "leaking" signals. This can interfere with radio transmissions, they argue.

The Civil Aviation authority has also raised concerns about powerline transmission technology, saying that it may pose a threat to guidance and landing systems.