Britain's 17 defunct nuclear submarines – seven of which are moored at the Rosyth naval dockyard in Fife – would have been dumped at sea under plans drawn up by the Ministry of Defence.

A secret MoD briefing found in the UK National Archives reveals the ministry's "technical preference" was to dispose of the radioactive hulks at sea without dismantling them. Dumping in this way, however, would raise "many environmental and other issues", it said.

Since Britain's first nuclear submarine, HMS Dreadnought, was taken out of service 30 years ago, the MoD has been trying to work out how to get rid of its reactor. Over the decades it has been joined at Rosyth by six other retired nuclear submarines, while another 10 have been tied up at the Devonport naval base at Plymouth in the south of England.

Since 2000 the MoD has conducted a series of prolonged public consultations on what to do with all the submarines' radioactive remains. The latest in the last year suggested that the reactor pressure vessels should be removed from the submarines at Rosyth and Devonport.

But what would then happen to them is still unclear. The MoD says it wants to work with the Government's Nuclear Decommissioning Authority on whether to build a new waste store, or to use current or planned stores at nuclear power station sites, like the one at Hunterston, Ayrshire.

The MoD's failure to ensure funding for decommissioning the submarines has been repeatedly criticised by its own internal nuclear safety regulator, Commodore David Langbridge. His 2011 annual report highlighted the lack of funding as a "risk to meeting Government policy".

The briefing unearthed from the UK National Archives by nuclear researcher Brian Burnell shows that the MoD has not always been so indecisive. Marked "confidential" on every page, it was prepared for Dr Dov Zakheim, a senior US defence official, ahead of a meeting in London, and sent in August 1981 by Newman Beaumont, an MoD section head.

How to get rid of defunct nuclear submarines was a "major issue", the briefing warned. "Disposal by cutting up and burial on land in the UK or long-term laying up at UK berths are not favoured due to practical, financial and environmental reasons."

It continued: "The MoD technical preference is for dumping the whole defueled submarine at sea, which is considered to be the cleanest, safest and most practical solution."

Defining the submarine as low-level radioactive waste, the briefing concluded, "should mean that there is no strictly legal bar to its disposal at sea but this raises many environmental and other issues".

The MoD dumped thousands of tonnes of radioactive waste in the English Channel and the northeast Atlantic from 1946 to 1983. In 1993 the MoD agreed to an indefinite ban on dumping radioactive waste at sea.

The Scottish Campaign for Nuclear Disarmament (SCND) accused the MoD of failing to adequately consider how to dispose of nuclear submarines. "Since this briefing was written, they have launched 11 nuclear submarines and are planning to build more," said SCND's co-ordinator, John Ainslie.

"Their callous suggestion that the vessels could just be dumped at sea illustrates how little thought they originally gave to this question."

Peter Burt from the Nuclear Information Service, which monitors UK military activities, welcomed the MoD's willingness to take decisions on scientific grounds rather that treating the oceans as a "giant dustbin". But he warned there was further to go.

"Key decisions on the Royal Navy's nuclear legacy will be made over the next few months and the MoD must listen very carefully to the views of the local communities."

Although it had considered sea disposal the best solution 30 years ago, the MoD stressed it had never dumped a submarine at sea. "As a responsible nuclear operator we are committed to dismantling and disposing of our decommissioned submarines in a safe, secure and environmentally responsible way that complies with legislation," said an MoD spokesman.

"Following a public consultation as part of the submarine dismantling project we are considering the responses, and decisions are expected to be announced in 2013."