The disaster was officially termed "the hand of God" but new evidence from previously classified government files suggests that a team of international scientists working with the RAF was experimenting with artificial rainmaking in southern Britain in the same week and could possibly be implicated.

Squadron Leader Len Otley, who was working on what was known as Operation Cumulus, has told the BBC that they jokingly referred to the rainmaking exercise as Operation Witch Doctor.

His navigator, Group Captain John Hart, remembers the success of these early experiments: "We flew straight through the top of the cloud, poured dry ice down into the cloud. We flew down to see if any rain came out of the cloud. And it did about 30 minutes later, and we all cheered."

The meteorological office has in the past denied there were any rainmaking experiments conducted before 1955, but a BBC Radio 4 history investigation, to be broadcast tonight, has unearthed documents recently released at the public record office showing that they were going on from 1949 to 1955. RAF logbooks and personnel corroborate the evidence.

Until now, the Ministry of Defence has categorically denied knowledge of any cloud-seeding experiments taking place in the UK during early August 1952. But documents suggest that Operation Cumulus was going on between August 4 and August 15 1952. The scientists were based at Cranfield school of aeronautics and worked in collaboration with the RAF and the MoD's meteorological research flight based at Farnborough. The chemicals were provided by ICI in Billingham.

Met office reports from these dates describe flights undertaken to collect data on cumulus cloud temperature, water content, icing rate, vertical motions and turbulence, and water droplet and ice crystal formation. There is no mention of cloud seeding.

But a 50-year-old radio broadcast unearthed by Radio 4 describes an aeronautical engineer and glider pilot, Alan Yates, working with Operation Cumulus at the time and flying over Bedfordshire, spraying quantities of salt. He was elated when the scientists told him this had led to a heavy downpour 50 miles away over Staines, in Middlesex.

"I was told that the rain had been the heaviest for several years - and all out of a sky which looked summery ... there was no disguising the fact that the seedsman had said he'd make it rain, and he did. Toasts were drunk to meteorology and it was not until the BBC news bulletin [about Lynmouth] was read later on, that a stony silence fell on the company," said Mr Yates at the time.

Operation Cumulus was put on hold indefinitely after the tragedy.

Declassified minutes from an air ministry meeting, held in the war office on November 3, 1953, show why the military were interested in increasing rain and snow by artificial means. The list of possible uses included "bogging down enemy movement", "incrementing the water flow in rivers and streams to hinder or stop enemy crossings", and clearing fog from airfields.

The documents also talk of rainmaking having a potential "to explode an atomic weapon in a seeded storm system or cloud. This would produce a far wider area of radioactive contamination than in a normal atomic explosion".

UK weather modification experiments at the time presaged current practice in the US. The idea was to target "super cool" clouds, and to increase the volume of freezing water vapour particles. Most methods involved firing particles of salt, dry ice, or silver iodide, into clouds, either from an aeroplane or from burners on the ground. The clouds would then precipitate, pulled down below freezing point by the extra weight of dense particles, thus making it rain sooner and heavier than it might have done. Significantly, it was claimed that silver iodide could cause a downpour up to 300 miles away.

Many countries now use the technology, which has considerably improved during the past 50 years.

But controversy still surrounds the efficacy of these early cloud-seeding experiments. In 1955 questions were asked in the Commons about the possibilites of liability and compensation claims. Documents seen by the BBC suggest that both the air ministry and the Treasury became very anxious and were aware that rainmaking could cause damage, not just to military targets and personnel, but also to civilians.

The British Geological Survey has recently examined soil sediments in the district of Lynmouth to see if any silver or iodide residues remain. The testing has been limited due to restrictions in place because of foot and mouth disease, and it is inconclusive. However, silver residue has been discovered in the catchment waters of the river Lyn. The BGS will investigate further over the next 18 months.

Survivors of the Lynmouth flood called for - but never got - a full investigation into the causes of the disaster. Rumours persist to this day of planes circling before the inundation.

 The Day They Made It Rain, Radio 4 , 8.30pm.