British UFO 'sightings' investigated by a secret branch of the MoD are soon to be revealed and officials are braced for a torrent of inquiries

Without warning, the orange UFO swooped toward them. The crew of the RAF Vulcan bomber banked hard and radioed they were being chased across the Atlantic by a large mysterious object. The incident was classified as a UFO sighting and the details were immediately locked away.

Now, 30 years later, the extraordinary encounter is among thousands of previously secret cases contained in the government's 'X-Files' that officials are to release in their entirety.

The cases, many from a little-known defence intelligence branch tasked with investigating UFO claims, will be published by the Ministry of Defence to counter what officials say is 'the maze of rumour and frequently ill-informed speculation' surrounding Whitehall and its alleged involvement with Unidentifed Flying Objects.

The public opening of the MoD archive will expose the once highly classified work of the intelligence branch DI55, whose mission was to investigate UFO reports and whose existence was denied by the government until recently. Reports into about 7,000 UFO sightings investigated by defence officials - every single claim lodged over the past 30 years - are included in the files, whose staged release will begin in spring.

The decision to release Whitehall's full back-catalogue of UFO investigations was taken last month after the Directorate of Air Space Policy, the government agency responsible for filtering sensitive reports, gave its permission to publish the biggest single release of documents in MoD history. Now the government fears a repeat of the unprecedented demand and the website crash experienced by the French national space agency in March when it released its own UFO files. Government IT experts are believed to have drawn up contingency plans to avoid a repeat scenario when Britain's dossiers are finally made public.

Among the first tranche of UK cases will be the official government files into the famous Rendlesham incident, dubbed 'Britain's Roswell' after the US incident when a flying saucer is said to have crash-landed in the New Mexico desert 60 years ago. On a foggy night in 1980 several witnesses reported a UFO apparently landing in Rendlesham Forest, Suffolk. Statements claimed the craft was covered in markings similar to Egyptian hieroglyphics and aliens emerged from it. Although a man later confessed to having staged the incident as a hoax, the files will clear up continuing speculation as to whether radiation was detected at the site after the event.

Another case reported to the intelligence branch DI55 - Britain's version of the 'Men In Black' - chronicles a series of reports sent to RAF Scampton, Lincolnshire, by the crew of a Vulcan bomber on exercise over the Bay of Biscay early on 26 May 1977. According to documents seen by The Observer, five crewmen, including the captain, co-pilot and navigators, watched 'an object' approach their aircraft at 43,000ft above the Atlantic. The mysterious craft then appeared to turn and follow their precise course from a distance of four miles.

Initially, the crew said the object resembled landing lights 'with a long pencil beam of light ahead' but as it turned towards them the lights suddenly went out leaving a diffuse orange glow with a bright fluorescent green spot in its bottom right-hand corner. Then, according to signals sent back to Scampton, the crew noted a mystery object 'leaving from the middle of the glow on a westerly track... climbing at very high speed at an angle of 45 degrees'.

The Vulcan's navigator recorded interference on his radar screen from the direction of the UFO which continued for 45 minutes as the plane headed back to Britain. On return to the UK, the camera film from the aircraft's radar was examined by RAF intelligence. They found a 'strong response' from the direction of the sighting. The UFO was captured as 'an elongated shadow' of a 'large-sized' object travelling at a similar height to the Vulcan. An intelligence report sent to the MoD the same day says the crew 'were unable to offer a logical explanation for the sighting'.

Although hailed as the complete disclosure of the UK's UFO files, questions are likely to remain over whether all available information will be made public. Despite the Vulcan sighting being investigated by DI55, no details remain in the file indicating what they found or what became of the radar film.

The disclosures are more likely, claim some experts, to lend credence to the theory that such UFO incidents were, rather than alien visitations, military activities such as missile launches, testing of prototype aircraft and other activities during the Cold War.

David Clarke, a lecturer in journalism at Sheffield Hallam University and author of Flying Saucerers: A Social History of UFOlogy, said: 'Something was definitely going on, but really these files show that the government did not know either. This release will be a source of disappointment or vindication for some, and embarrassment for others.

'Conspiracy theorists who believe that the various governments of the world are hiding secrets about the "reality" of aliens will see this as another whitewash effort by the MoD and will probably continue their self-sustaining "campaign for the truth", when the truth will in fact now be "out there".'

UFO researcher Joe McGonagle said: 'There will always be a hard core who believe these files were prepared for release and that there is a secret department within the military who has a separate stash of files that have not been disclosed.'

UFOs remain one of the most popular subjects for Freedom of Information requests and the release is certain to generate a massive response from the public when the files are placed in the National Archives. Clarke, who has lodged hundreds of FoI requests, recently discovered that the government was considering destroying the 24 files created by DI55 because they were contaminated by asbestos. Not only were the UFO records polluted, but a total of 63,000 files estimated at between six to 12 million pages - most of them classified as secret - were facing the same fate. Having admitted the existence of the problem to Clarke, the MoD opted to instigate a £3m project digitally to scan the files before they were destroyed. Scanning of the 24 contaminated UFO files owned by DI55 was completed last year, although it is understood that names of officials in the reports will be removed.

Although the government remains reticent to discuss its intelligence work on UFOs, it is known that DI55 has been hot on the trail of flying saucers since the Sixties. Experts admit that they work closely with the security services MI5, MI6 and GCHQ to collect and assess evidence of potential threats to Britain.

The decision by the UK to open its files could lead to the US government following suit. A group of former pilots and government officials recently urged the Pentagon to reopen investigations into claims of UFO sightings.

UFO claims

1980 Rendlesham Forest, Suffolk. US servicemen claim to have seen an alien craft and its landing site.

1984 Minsk, USSR. Aeroflot pilots say they are pursued by a glowing shape.

1989 Bonnybridge, Scotland. Fire crew report objects rushing towards them before veering away at the last moment.

1990 Brussels, Belgium. Two F-16 fighter pilots recount being engaged in 75-minute mid-air chase with a UFO.

· This article was amended on Sunday 20 2008. In the sentence in the article above: 'Although the government remains reticent to discuss ...', we should have said reluctant, not reticent. This has been corrected.