A cargo vessel was heavily overloaded and had been subject to ''cost-cutting measures'' when it sank off the British Isles, killing six crew members, an investigation has found.

The Russian-crewed Swanland, which sailed under a Cook Islands flag, had not been properly maintained and was severely weakened by corrosion in the two years between its intermediate survey and the accident in the Irish Sea on November 27, 2011.

The whole incident could have been avoided, investigators said.

Two of the crew - Second Officer Roman Savin, 27, and Able Seaman Vitaly Karpenko, 48 - were airlifted to safety by an RAF helicopter co-piloted by the Duke of Cambridge as the ship sank. Chief Officer Leonid Safonov, 50, was pronounced dead after his body was recovered from the sea shortly afterwards.

The bodies of the five remaining crew members have never been discovered.

A report by the Marine Accident Investigation Branch (MAIB) today outlined several problems with the Swanland, which was carrying nearly 3,000 tonnes of limestone when it experienced a ''structural failure when heading directly into rough seas and gale force winds'' while on passage from Llanddulas in Wales to Cowes on the Isle of Wight.

The ship sank shortly after the first mayday call.

A spokesman for the MAIB said: ''This was a tragic and avoidable accident that cost the lives of six crew.

''Worldwide, between 2002 and 2011, nearly 250 cargo vessels foundered causing the deaths of more than 800 seafarers - statistics that bring into sharp focus the need for action to be taken to improve the safety of general cargo vessels.''

The investigation identified that the major factors contributing to the structural failure included the way the cargo was loaded - putting pressure on the midships section - and a lack of repairs in recent years.

Other contributing factors included non-compliance with an international maritime code, a lack of effective safety management and the ''financial pressures of operating this type of vessel in the current economic downturn''. The investigation also identified several safety issues concerning the immersion suits and life jackets on the vessel.

In its report, the MAIB said the owners Swanland Shipping Limited decided to change the vessel's classification society from Lloyd's Register (LR) to the International Naval Surveys Bureau (INSB) in 2009, reducing the fees paid by about 30%.

The report acknowledged the transfer as a ''money saving'' initiative, and added: ''Although the savings made on the costs of survey and audit fees would have been immediate, it is recognised that many ship owners also enter their vessels with non-IACS (International Association of Classification) societies expecting that the surveys and audits conducted will be less robust.

''In effect, significant long-term savings are possible through reduced cost of repairs and the rectification of deficiencies. This was certainly the case for Swanland.''

The report also highlighted how Swanland's crew did not regularly conduct ''abandon ship'' drills and were not provided with regular opportunities to practice mustering.

The report added: ''There is no justifiable reason why the safety record of general cargo vessels should be allowed to lag behind other vessel types, such as bulk carriers, without vigorous attempts being made to redress the balance.

''It is hoped that the loss of Swanland and her six crew will be a catalyst for the work already being undertaken by the International Maritime Organisation to tackle the global issue of general cargo ship safety.''

The MAIB has issued a number of recommendations including highlighting the need for all vessels carrying solid bulk cargoes to comply with regulations.

The bodies of ship master Yury Shmelev, 44, chief engineer Geeadiy Meshkov, 52, second engineer Mikhail Starchevoy, 60, Able Seaman Sergey Kharchenko, 51, and ship's cook Able Seaman Oleg Andriets, 49, remain lost at sea.