The top secret colour photographs, said to show a flying saucer above the Scottish Highlands in August 1990 - the so-called Calvine Incident - were set to be released in the New Year.

However, they have now been blocked for a further 50 years. Here, former MoD official Nick Pope, who previously ran its UFO project, tells The Sun the inside story...

In the cult sci-fi series The X-Files, Fox Mulder has a poster of a UFO on the wall of his basement office. Underneath are the words "I want to believe".

In the Ministry of Defence office, which served as the nerve centre of the UK’s UFO project, we had something very similar. But our picture was real.

Most UFO photos are either obvious fakes, or blurry and indistinct – a vague light in the night sky, or a fuzzy dot in the distance.

Not this one. It was up-close-and-personal, had been taken in broad daylight, and showed a large diamond-shaped craft.

I soon got the story out of my predecessor and read the file myself. It was an extraordinary tale: two men had been out hiking near Calvine in Scotland.

Suddenly, they’d seen a massive UFO hanging in the sky above their heads, silent, motionless and menacing. Awestruck, they shot off six photographs before the object accelerated away at immense speed – vertically!

The shell-shocked witnesses sent the photos to a Scottish newspaper and a journalist contacted the MoD press office, looking for a comment.

Somehow - perhaps using a D-Notice or perhaps using some real-life Men-in-Black trickery - someone at the MoD managed to extract all the photos and the negatives from the newspaper, who never got them back.

The MoD’s technical wizards leapt into action. The images were enlarged and analyzed, using the full resources and capabilities of intelligence community specialists.

Even now, years after these events, I can’t discuss the details of this process, as so much of the information is top secret.

'Unlike any conventional aircraft & nearly 100 feet in diameter'

The analysis was nothing short of sensational. The photos hadn’t been faked.

They showed a structured craft of unknown origin, unlike any conventional aircraft. There was no fuselage, no wings, no tail, no engines and no markings of any sort.

Because the photos had been taken in daylight with the surrounding countryside visible, MoD boffins could make some calculations about the mystery object’s size. It turned out to be nearly 100 feet in diameter.

An enlargement of the photos revealed two military jets in the background. It wasn’t clear if they were escorting the UFO, trying to intercept it, or whether their presence was coincidental and the pilots had been too far away to see it.

'Did someone sabotage the investigation?'

My predecessor had undertaken the investigation and I was staggered to learn that it hadn’t proved possible to trace the aircraft.

This was unprecedented and suggested that someone inside the MoD had sabotaged the investigation and blocked the UFO project from getting to the truth. Dark forces were at work.

The MoD’s standard line on UFOs was that the phenomenon was of ‘no defence significance’ – a meaningless Whitehall soundbite that meant whatever we wanted it to mean.

At best it was misleading and at worst, it was a downright lie.

We consistently played down the true level of our interest in UFOs, telling parliament, the media and the public that the subject was of little interest, while all the time, behind closed doors, we struggled to make sense of cases like the Calvine incident.

Despite an extensive investigation, we never found a definitive explanation for what was seen at Calvine.

Of the several hundred sighting reports we received each year, most turned out to be misidentifications of satellites, meteors, Chinese lanterns and other ordinary objects and phenomena.

But around five percent of the cases remained unexplained.

'I can reveal the sensational conclusion to our secret briefing'

We didn’t assume these unexplained cases were extraterrestrial, but neither did we rule out the possibility and by the mid-Nineties a believer faction had emerged at the MoD. This led to some extraordinary scenes.

One time my boss and I walked over to another building in Whitehall. It was the headquarters of the Defence Intelligence Staff – not as well-known as MI5 or MI6, but part of the intelligence community.

While the section where I worked had the policy lead for UFOs and handled the investigations, these were the ‘spooks’ who gave us scientific and technical advice.

My boss was a sceptic, but our military intelligence briefer that day wasn’t. He pulled out a copy of one of the Calvine photos from a folder.

Most of the details of our conversation are classified and must remain secret. While I took early retirement from the MoD in 2006, the Official Secrets Act binds me for life. But I can reveal the sensational conclusion to our top secret briefing.

Summing up, the intelligence officer ran through the possibilities. Gesturing to the left with his finger, he said the object in the photograph wasn’t Russian. Jerking his hand to the right, he said it wasn’t American. He looked at us intently and said that only left one other possibility.

He pointed straight up. My boss and I couldn’t help looking up too. Then we looked at each other and then we looked back at the intelligence officer. His face was inscrutable.

Nothing further was said and my boss and I walked back to our office in silence.

'A sceptic versus believer dogfight was raging'

The photos could cause wonderment, but they could cause controversy too.

One time, they nearly caused a diplomatic incident that threatened to unravel the UK’s Special Relationship with the United States.

By the mid-Nineties a bitter struggle had erupted within the MoD in relation to UFOs. A sceptic versus believer dogfight was raging.

In relation to the Calvine photos, the only remotely possible skeptical theory was that the object was a secret prototype aircraft or drone.

We knew what we did and didn’t have when it came to such things, so realistically, that only left the Americans.

At the same time as this row was raging, speculation arose that the Americans had developed a secret prototype aircraft codenamed Aurora – a hypersonic replacement for the iconic SR-71 Blackbird.

We asked the US authorities if they’d been testing such an aircraft over the UK, but received firm denials. With the Calvine photos in mind, some MoD officials didn’t believe the US assurances, so asked again.

The Secretary of the Air Force, Donald Rice, was ‘incensed’ by the questioning and the implication that he’d lied to the US Congress when he told them Aurora didn’t exist.

Our Air Attaché at the British Embassy in Washington wrote to senior RAF officers, pleading with them to defuse the row.

As a result of this diplomatic bust-up, my Head of Division removed the Calvine incident photo from our office wall.

He, like my immediate boss, was a sceptic, and clung to the belief that the object had to be American, because the concept of extraterrestrial visitation was too terrible a truth for him to face.

He locked it away in his safe, and it’s rumoured that he put it through the shredder, out of loyalty perhaps, or maybe fear.

'I’ll believe it when I see it'

Plenty of other copies survived and when I came out of retirement in 2008 to help publicise the declassification and release of the MoD’s UFO files, I was looking forward to the Calvine photos seeing the light of day. Sadly, it wasn’t to be.

When the relevant files were released, the photos were missing.

All that remained were some poor-quality black and white photocopies of a line drawing of one photo.

Intelligence analysts use line drawings in their work, but to the media and the public, they look cartoonish. It was almost as if the MoD wanted to ridicule the subject.

A few years ago I teamed up with a graphic artist in Los Angeles and we reconstructed the photo for a TV show, using the line drawing and my memory as a guide. The result was spot-on, but it’s still not the real thing.

I’ll be delighted if the Calvine photos are released in the New Year, but I’m not getting my hopes up. I’ll believe it when I see it.

Nick Pope worked for the Ministry of Defence for 21 years. From 1991 to 1994 he ran their UFO project