The munitions dump in the Beaufort's Dyke trench, off Stranraer - which holds more than 1 million tons of bombs, rockets and shells, including 14,000 tons of rockets with phosgene poison gas warheads,is seven times larger than previously thought.
The dump has been in use for much longer than previously admitted - perhaps 50 years. The MoD has now said the Ministry of Agriculture, Fisheries and Food will carry out an underwater television survey in 1996 to look for leaks or other contamination.
The true size of the underwater dump, is revealed in a letter to Duncan Shaw, chairman of the Irish Sea Forum, a Liverpool based watchdog. The MoD has said that the dump, which is 300m deep, may have been used for surplus munitions from 1920 to 1976.
Between 1945 and 1948, after the Second World War,135,000 tons of munitions were dumped there, in a tightly defined area. However, the MoD's letter reveals that dumping may have started after the First World War, and was probably not restricted to the area laid down in a Notice to Mariners in 1945.
It also says that other organisations may have used the area to dump toxic waste. Dumping continued at a rate of 20,000 tons a year until the late 1950s, and then reduced to 3,000 tons a year. The last dump, of 40mm Bofors anti-aircraft shells, took place in 1976.
The MoD also admits that in 1945 14,000 tons of 5-inch artillery rockets with phosgene gas war-heads were dumped in Beaufort's Dyke. Yesterday, Dr Shaw said these were unlikely to be dangerous, as phosgene decomposes easily in water.
More dangerous chemical weapons were disposed of in deep water further out to sea.The letter also details operation Sandcastle, in 1955-56, in which 71,000 quarter-ton German bombs filled with the nerve gas tabun were loaded into the hulks of three merchant ships and dumped between 2,000 and 2,4000m down, 80 miles north-west of Ireland.
Nerve gas was not invented until the late 1930s, and if chemical weapons were dumped at Beaufort's Dyke before 1945, they would have contained materials which decompose harmlessly in water, Dr Shaw said.
However, Paul Johnston, an explosives expert at Exeter University, told the Scotsman newspaper that the explosives could still be active and that shifting currents could disturb the acres of submerged munitions and that drums containing chemicals could corrode and leak.