Minutes of secret meetings released under freedom-of-information law reveal the MoD was worried about breaching an inter-government agreement on marine pollution by firing depleted uranium (DU) tank rounds into the sea from a military range at Dundrennan near Kirkcudbright.
But officials found a way round the problem, by claiming the munitions were not being "dumped" in the sea, but "placed" there. This is despite the fact attempts to retrieve them have failed, and their locations are unknown. DU is toxic and radioactive and has been linked to increases in cancers and birth defects in Iraq, where it has been used as a weapon. It has also been linked to health concerns among members of UK armed forces exposed to the shells.
Campaigners have accused the MoD of resorting to "semantic trickery" to justify its plans to dump more DU weapons in Scottish waters.
In the past 30 years, over 6,700 shells have been fired from the range, containing nearly 30 tonnes of DU. They pierce canvas targets on the cliffs, then plunge in the sea.
At an MoD meeting in June 2004, an official was minuted suggesting there could be "a future problem" firing DU into the sea. The Convention for the Protection of the Marine Environment of the North-East Atlantic, known as the OSPAR convention, agreed by 15 governments, including the UK, said "it was illegal to dump waste into the sea", he warned.
According to the minute, this provoked "discussion surrounding the wording". But another official said there was no problem because the MoD's interpretation was "the projectiles were placements not dumping". The 1992 OSPAR convention says dumping does not include "placement of matter for a purpose other than the mere disposal thereof".
Aneaka Kellay from the Campaign Against Depleted Uranium said: "The Scottish public will struggle to understand how the MoD thought they could evade their legal and moral responsibility not to pollute the sea by calling this a 'placement'.
"However the MoD name their firing programme, the fact remains they have purposefully released nuclear waste into the Solway Firth, with little idea of how this will affect the marine environment."
She called on the MoD to retrieve the DU rounds it had fired in the sea, and to refrain from firing more "as we seek legal advice on this glorified form of dumping."
Katy Clark, the Labour MP for North Ayrshire and Arran, said: "The legal basis on which the test-firing has occurred is open to serious questioning."
An MoD spokeswoman said: "All testing is in accordance with procedures agreed with the Environment Agency and the Scottish Environment Protection Agency."
The Scottish Government said it was "strongly opposed to the testing by MoD of DU shells on Scottish soil and in Scottish waters".