A helicopter-carrying merchant ship that sank with the loss of 12 men after being hit by two Exocet missiles in the 1982 Falklands conflict was unarmed and unprotected because Ministry of Defence lawyers feared that it was illegal to fit a commercial vessel with weapon systems, according to newly released classified documents.

The container ship Atlantic Conveyor, which had sailed to the South Atlantic just six days after being requisitioned by the MoD, was struck on May 25, causing devastating fires and explosions on board – a storage section filled with cluster bombs and kerosene blew up.

It was one of the biggest-impact attacks by Argentine Exocet-armed Super Etendard bombers because the Atlantic Conveyor was carrying four Chinook and seven Wessex helicopters, all of which would have played a crucial role in ferrying British troops across the Falklands as part of the campaign to liberate the islands.

The Argentinians were hoping to target one of the two Royal Navy aircraft carriers but the missiles homed in on the 14,950-tonne merchant ship.

Now MoD documents released under the Freedom of Information Act have revealed that the Atlantic Conveyor was not protected by her own defensive systems. Nor did she have a dedicated frigate escort equipped with an antimissile system because there were not enough war-ships available to guard the vessel.

Navy chiefs had decided that the ship was not “a high-value unit” so she was not even fitted with chaff, the system which fires aluminium strips into the air to form a decoy target to distract an approaching missile.

Although there had been little time to convert the container ship for wartime duty, the released document recorded: “There was controversy in the MoD over the decisions whether or not to arm auxiliaries and whether such a move would be legal.”

The MoD document concluded: “ Atlantic Conveyor was not fitted with self-defence weapons and chaff dispensers before sailing from the UK. The [Navy] Board has been unable to establish the reasons for this omission, other than the short timescale involved and controversy over the legality of arming auxiliaries.”

The container ship had successfully completed one of her principal functions before she was attacked on May 25. Apart from helicopters, Atlantic Conveyor was also carrying eight Sea Harriers and six GR3 Harriers. These were all transferred to HMS Invincible and HMS Hermes, the aircraft carriers.

However, Atlantic Conveyor was kept within range of Argentine bombers because ground commanders needed instant access to the helicopters that she had on board.

With frigates and destroyers tied up protecting the two carriers and the beach-head at San Carlos where the British troops landed, as well as providing naval gunfire support, the report says: “the Commander Task Group [Rear Admiral Sandy Woodward] did not have sufficient escorts, particularly those fitted with Seawolf [antimissile system] to go round”.

The devastating consequences were recorded in a separate document that has also been released.

A “narrative” logbook-style account reporting the events that transpired on May 25, 1982, revealed: 1940: Air-raid warning RED (no threat direction) – assumed to be West. Emergency stations piped, siren sounded as supplementary alarm.

1942: Missile approach sighted. ACO [ Atlantic Conveyor] hit by two Exocets . . . 10-12ft above waterline. Missiles entered C cargo decks. Eight people known to be between decks at this time. “Hit the deck” called over ship’s broadcast.

1952: Thick black smoke reported on all A deck.

2000: Several explosions from aft on C deck. Small arms ammunition and phospher grenades ditched. Three men succumbed to smoke.

2005: Fire assessed out of control and risk of fire spreading forward to magazine area.

2010: Decision taken to abandon ship.

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