Documents discovered at the Public Records Office showed that during the 1950s, roughly two tonnes of concrete-encased metal drums, filled with laboratory rubbish and luminous paint, were dropped into the Irish Sea's Beaufort Dyke, 300 metres deep and 10 kilometres off the Scottish coast and close to busy shipping lanes.
Government spokesmen insisted that the waste itself, classified as low- level and intermediate-level, would pose no risk to health. However MPs and pressure groups yesterday insisted that it showed flaws in the accountability of the Ministry of Defence, the Scottish Office and the Ministry of Agriculture, Fisheries and Food, which has taken responsibility for monitoring the waste. The seven-mile-long strip of Beaufort Dyke has been used as a munitions dump by the Ministry of Defence since the 1920s. The previous government admitted that at least one million tonnes of bombs, rockets and shells, including some with chemical warheads, were dumped there. But since 1984, ministers had denied that any radioactive waste was disposed there, based on data from an independent report which said it was dumped in the mid- Atlantic in water 2,000 metres deep.
But yesterday, the Scottish Office confirmed the report, and said that a formal Commons announcement - probably in response to a written parliamentary question - will follow either today or later this week.
John Large, an independent nuclear consulting engineer, said: "This is a very serious issue. The nuclear industry describes waste as being low, intermediate or high level. But these terms apply to waste stored in controlled conditions on land. As soon as you unzip that can underwater it doesn't matter. It is like a leaking teabag and the uptake would be a slow and gradual process.
"Here you have the risk of radioactivity being taken up by plankton and then by fish where it ultimately ends up on the landing slab at Grimsby."
Details about the dumping of the waste, from private companies including defence contractor Ferranti, have only recently emerged, according to a Scottish Office spokesman.
In contacts with London yesterday, Ireland's natural resources minister, Michael Woods, expressed "deep concern" at the revelation.
Eamon Gilmore, the junior marine minister in the outgoing Irish government, accused the former Tory administration of misleading the Dublin authorities over the issue.
He said that while he was in office he had been assured by the then British government that the Beaufort Dyke dump contained no nuclear waste. Radioactive emissions from Sellafield have been at the centre of lengthy controversy in Ireland amid claims of unusually high cancer rates in the Dundalk area on Ireland's east coast.