An official dossier of 15 documents, which has been dubbed "Britain's X-Files", has been released by the UK Ministry of Defence (MoD) and published in paper form by the National Archives – but one expert has suggested three withheld files will lead to accusations of a cover-up.

At the end of last week the National Archive released the files, outlining MoD investigations into mysterious incidents and unidentified flying objects (UFOs). They were not digitised and can currently only be viewed in person or by buying them and waiting for delivery.

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UFO watchers and those who closely follow the phenomenon of alien life have been waiting for the files to be released, hoping they will reveal secrets from deep within the British government.

Nick Pope, a former MoD expert on the subject, said withholding files will only fuel conspiracies.

"It seems that The National Archives has released 15 of the remaining 18 Ministry of Defence UFO files, marking the final stages of what's been a nine-year long project to declassify and release the MoD's entire archive of UFO files," Pope told The Sun.

"Unlike all such previous releases of UFO files, I wasn't pre-briefed on the release date.

"The release of these final files has been extensively delayed and at one stage they were sent from the MoD to the National Archives, but then sent back to the MoD."

He continued: "Embarrassment about this delay may explain why - unlike releases of previous batches of MoD UFO files - there hasn't been a proactive media release. Accordingly, I suspect the UFO and conspiracy theory community will say that these files have been 'snuck out'."

What's in the 'X-Files'?

Many sky-watchers hoped the fresh files would shine a light on a long-running mystery that has become known as the Rendlesham Forest incident – or Britain's Roswell. This "sighting" happened in 1980 at a military base near Suffolk, with some reports of a "triangular shaped craft."

But unfortunately for those chasing little green men, Pope told British tabloid press he did not believe the MoD files contained a smoking gun. He said some documents likely reference the Rendlesham Forest case but will probably be duplicates of already-released material.

He told The Express: "The release of these real-life X-Files shows that the UFO mystery lives on and that there's still huge interest in this fascinating topic. These files will give people a revealing insight into this bizarre and intriguing aspect of Ministry of Defence business.

"Sadly, there's no 'smoking gun' in these files that will prove we've been visited by extra-terrestrials, but there are plenty of intriguing UFO reports, as well as policy papers explaining how the MoD handled this subject.

"The lack of a smoking gun and the fact that these files seem to have been slipped out without a formal media announcement is bound to start some conspiracy theories, and I know that many people believe the 'good stuff' is being held back.

"Furthermore, the fact that some files haven't been digitised, can't be downloaded, and can only be viewed in person at the National Archives will doubtless also generate conspiracy theories.

"After this, there are only three more UFO files still to be released."

Pope said these should be made public "later this year."

This week (26 June), The Sun newspaper, which has seemingly sent a reporter to analyse the archived documents, reported they reveal how a UFO was spotted on a military radar for hours over the Skegness coast in 1996 – with a RAF reportedly being told not to investigate further.

The referenced document, which does not reveal an author's name, contains snippets of news from the time – one story titled "UFOS expose 9-5 defence danger".

It allegedly included eyewitness accounts from police, claiming to have viewed "flashing red, blue and white lights" in the sky.

The news comes after a chief scientist at Nasa was forced to refute claims from Anonymous, the loose collective of hackers, which said the agency was on the cusp of revealing alien life.

"Contrary to some reports, there's no pending announcement from Nasa regarding extra-terrestrial life," wrote Nasa's Thomas Zurbuchen on Twitter.

He added: "Are we alone in the universe? While we do not know yet, we have missions moving forward that may help answer that fundamental question."

Contrary to some reports, thereâs no pending announcement from NASA regarding extraterrestrial life. — Thomas Zurbuchen (@Dr_ThomasZ) June 26, 2017

Nick Pope has long investigated claims of extra-terrestrials and UFOs, but has consistently denied his own life follows the tropes put forward by science fiction movies.

"How do the MoD's real-life Mulders and Scullys compare to their fictional counterparts? Having done the job myself, from 1991 to 1994, I'm sorry to have to report that it's not quite as glamorous as people might suspect," he wrote in the Guardian back in February 2010.

"There's no running around dark warehouses with guns and torches. It's more a case of asking colleagues in the RAF to check the radar tapes and writing polite letters back to members of the public, stating that most UFOs turn out to be misidentifications of ordinary objects."

It seems that, for now at least, the truth is still out there.