Newcastle United supporter Mike Gibbard said he stumbled across the documents on his way to a game in the city on March 16.

The army papers -- some marked "OFFICIAL - SENSITIVE" -- were spilling from a black bin bag and "spread all the way up the road", Gibbard said on BBC Radio Newcastle on Friday.

"I peered down and started to see names on bits of paper and numbers, and thought 'what's that?'" he said.

The BBC said the papers -- many of them torn -- included details about soldiers' ranks, emails, shift patterns, weapon issue records, and access information for military facilities.

One sheet was headed "armoury keys and hold IDS codes," an apparent reference to an intruder detection system.

The broadcaster said several documents appeared to relate to Britain's largest army garrison, Catterick, but security consultant Gary Hibberd told AFP the information risked compromising wider national security.

"The impact and scale of this is quite big -- it's not just a blunder. This will be investigated within highest levels of the military," Hibberd said.

A Ministry of Defence spokesperson said: "We are looking into this urgently and the matter is the subject of an ongoing internal investigation."

They confirmed that "documentation allegedly relating to the department was recently handed in to the police".

Northumbria Police told AFP that officers had been alerted to the find in the Scotswood district and had since passed on the papers to the defence ministry.

A spokesman for Prime Minister Keir Starmer said "appropriate action will be taken in response to any potential information breach".

UK government guidelines state that sensitive documents should be incinerated, pulped or shredded -- but confidential papers have ended up in several unusual places in the past.

One of the most high-profile cases took place in 2008, when a British civil servant left a folder of intelligence documents marked "Top Secret" on a train seat in London.

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IDS