Sniffer dogs at the Maze prison were barely able to detect Semtex - because a shortage of the explosive meant they couldn't be trained properly.
At a time when the almost odourless plastic explosive was the weapon of choice for republican terrorists, the security forces struggled to get their hands on it for training purposes.
A senior figure from the Northern Ireland Office's prisons and security operations division said: "The unfortunate truth is that some two to three weeks after training, our dogs are no longer able to identify Semtex as they receive no practice whatsoever in its detection.
"Our flank is completely exposed from this point of view, and is set to remain so unless steps are taken to iron out whatever barriers there are to the release of quantities of Semtex to interested agencies."
Former Libyan dictator Muammar Gaddafi supplied several tonnes of Semtex in shipments to the IRA during the 1970s and 1980s.
It was used in attacks such as the 1987 Enniskillen Poppy Day explosion and the 1988 Ballygawley bus bombing.
But the Prison Service and RUC were concerned about a shortage as Semtex could not be bought on the open market.
And, although the Army was making regular seizures at the time, legislation made it difficult to obtain.
The NIO official added: "With any explosive (or drug), once the initial course is completed, handlers are issued with a small sample of the substances to enable them to keep their dog familiar with the smells involved.
"The secret of success is regular (almost daily) training exercises, allowing the animal regular successes and rewards to maintain its motivation. Handlers are licensed accordingly.
"Despite explaining our case carefully to the RUC, I have been unable to obtain the supply of Semtex H explosive to enable our handlers to maintain the standard achieved on initial and refresher training (at six monthly intervals) courses.
"The inability of the RUC to provide us with the substances is not due to any lapse on their part. As the correspondence shows they do not have enough for their own, never mind for other agencies.
"Quite simply, because Semtex is an illegal substance, there is no legal source for them to approach to obtain it."