They told the pensioner and her friend that distress signals from ships at sea had been traced to her house. Lifeboats and air-sea rescue helicopters had been launched on several occasions but coastguards had drawn a blank. Nothing was found ... except that her house was the source of the signals.

“It was incredible,” said Mrs Donaldson, 67, of Plymouth, Devon. Her friend thought that she had been caught out by the television licence detector van.

The two officials identified the source of the radioed SOS calls as Mrs Donaldson’s digital television box. The “military in distress” mayday signals were picked up by satellite and intercepted by the RAF Aeronautical Rescue Co-ordination Centre at Kinloss, Scotland.

They immediately alerted coastguards in the area from which the distress call was coming.

Twice in recent weeks the coastguard at Lee-on-the- Solent launched fullscale search and rescue operations. Two lifeboats and a helicopter were scrambled and for three hours combed 20 miles of coastline around Portsmouth harbour, at a cost of more than £20,000.

Then it happened again and a two-hour search was launched. Twice they found nothing amiss and all the rescue crews returned to base.

Last night the cause of all the distress was revealed as Mrs Donaldson’s Freeview digital television receiver. Michael Mulford, an RAF spokesman, said: “This is very unusual. It’s a complete freak, and the odds of a digibox sending out a 121.5 signal must be astronomical.”

A spokesman for Ofcom, the broadcasting regulator, said that digital boxes were designed only to receive signals, not to transmit them. “They shouldn’t be sending out signals at all, let alone maydays,” he said.

At home Mrs Donaldson said: “I still can’t believe that little box sparked all this. I came back from watching a film to find two men holding a massive antenna. My friend thought I hadn’t paid my television licence or something. It was incredible, like a dream.”

She added: “I don’t think I’ll be getting a new one. I’d hate to cause any more bother”.

The frequency used by the digital Freeview set-top box was identical to that dedicated to emergency distress beacons.

The beacons are carried by ships, yachts and aircraft, and are activated when they come into contact with water, sending a signal that identifies the vessel and its location.

There are more than ten million Freeview boxes in the country, costing as little as £30 each, but Ofcom officials believe that it may be only a small batch that are faulty and can send out the mayday signals.

The Ofcom spokesman added: “Digital boxes are not meant to transmit any signals at all. The boxes are with our testers now and we will get the results in a couple of weeks. We suspect some components have malfunctioned.

“Apparently any device capable of receiving a signal can also send a signal if it malfunctions. To the best of our knowledge these are the only two out of millions of Freeview users in the UK to have experienced this problem.”

A spokesman for the ARCC at RAF Kinloss, which handles nearly 2,000 rescue missions a year and last year saved 1,295 lives, said: “It is almost unbelievable that this bit of equipment, which is in thousands of homes, was transmitting on the distress frequency.”

The mayday frequency of 121.5mhz broadcast by the set-top box is changing in 2009 to 406mhz digital to comply with international law.

Explore UK News

• Crime News

• Education News

• Health News

• Science News

• Scotland News

Times Recommends

• Anne Darwin's performance of a lifetime
• Renaissance for canals as new boaters come aboard
• Case study: forced to marry a cousin at 17