He claims to have seen spaceships soaring across the city's skies and crop circles 300ft-wide, so it will take more than an official report to convince this Oxford UFO expert aliens don't exist.

A confidential four-year study by the Ministry of Defence, made public under the Freedom of Information Act, concluded there is no proof of alien life forms.

But Michael Soper, pictured, a member of the Contact International UFO research group, has spent decades of his life studying unidentified flying objects and believes the MoD's report does not throw any doubt on his findings.

Mr Soper, of Ouseley Close, New Marston, who says he first saw a UFO in 1959 over the Malvern Hills in Worcestershire, said: "Obviously, the MoD can't admit aliens exist.

"Even if they knew they did, they could not admit to it. This is the only kind of report they could produce. I don't agree with most of it."

The report, undertaken by the Defence Intelligence Staff, part of the MoD, concludes that sightings of UFOs can be rationally explained and that meteors and their "well-known effects" are responsible for "some unidentified aerial phenomena".

But Mr Soper insisted this could only explain about five per cent of the cases he had encountered.

He said: "You would need to have a strange mentality to believe the conclusions of this report. I have read thousands of witness reports and seen all kinds of evidence and the MoD's conclusions do not come anywhere near to explaining them."

Mr Soper, a researcher and mathematician, said he had pictures of an object he believes to be an alien craft which he spotted on a photo taken of a cloud near Banbury. In 1995, he took a picture of a spherical object above the Co-op store in Marston and has also investigated crop circles near Garsington. He said: "I've heard petrol engines just cut out when a UFO has flown overhead. I've seen pennies burned after UFOs have landed on them.

"There are multiple witness reports. The MoD's report shows there is a case to answer, if they feel the need to produce a 400-page report. We were told there was no study being undertaken, but in actual fact a four-year study was being carried out.

"It is a bittersweet victory for us. They have admitted there is something to explain."

The report, entitled Unidentified Aerial Phenomena in the UK, was completed in 2000 and stamped "Secret: UK Eyes Only".

Only a few copies were produced and the author's identity has been protected.