Sniffer dogs at the Maze prison were barely able to detect Semtex during the late 1980s, top secret files have revealed.
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 (NIO) prisons and security operations division said: "The unfortunate truth is that some two-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, the Ballygawley bus bomb in 1988 and about 250 other booby-trap bombings.
But the Prison Service and Royal Ulster Constabulary police force 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 in this field 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 sources for them to approach to obtain it.
"The puzzling aspect of this situation is that the Army can and do obtain Semtex following incidents and finds both here and on the mainland.
"Why the RUC cannot obtain supplies from the Army, when they are in the front line facing devices made from Semtex is something of a mystery to me."
The batch of correspondence, marked confidential, is contained among files from 1988 which have been released by the Public Records Office Northern Ireland.
It reveals the issue was first raised by a dog handler at the high security jail which housed hundreds of paramilitary prisoners during the Troubles.
In a handwritten letter to the jail governor, dated July 1988, the prison officer said : "Semtex is produced by the Eastern bloc and is widely available to terrorist organisations throughout the world including, as has been shown in a number of devices disarmed recently the PIRA.
"Given its availability to the Provisionals it is therefore likely that any explosive substance discovered by an explosive search done within the jail it is likely to be Semtex.
"I therefore request that a sufficient amount of Semtex be made available to allow for continuation training to ensure that our explosive search dogs remain familiar with Semtex."