A yacht that capsized leaving four UK sailors missing in the North Atlantic may have been affected by structural weaknesses, a report has said. The Cheeki Rafiki crew had diverted the vessel after it began taking on water last May but contact was then lost. Days later the hull of the 40ft yacht was found with its life raft still on board. There was no sign of the crew. The vessel may have been weakened by previous groundings and subsequent repairs, the MAIB report has said. Skipper Andrew Bridge, 22, from Farnham in Surrey, was lost in the disaster along with crew members James Male, 22, from Romsey, Steve Warren, 52, from Bridgwater in Somerset, and Paul Goslin, 56, from West Camel in Somerset. They were returning from Antigua Sailing Week to Southampton when they capsized approximately 720 miles (1,160km) east-south-east of Nova Scotia in Canada. An air and sea search mission covered hundreds of miles east of Cape Cod, Massachusetts, but was eventually called off. The Marine Accident Investigation Branch (MAIB) said the cause remained "a matter of some speculation" in the absence of survivors and material evidence. "However, it is concluded that Cheeki Rafiki capsized and inverted following a detachment of its keel," says the report. "In the absence of any apparent damage to the hull or rudder other than that directly associated with keel detachment, it is unlikely that the vessel had struck a submerged object. "Instead, a combined effect of previous groundings and subsequent repairs to its keel and matrix (or lining) had possibly weakened the vessel's structure where the keel was attached to the hull." It said one or more keel bolts may have deteriorated and it was "probable that the crew were fatigued and their performance was impaired accordingly". Since the disaster the yacht's operator, Stormforce Coaching Ltd, said it had made changes to its internal policies and taken a number of actions aimed at preventing a recurrence. The Maritime and Coastguard Agency has been working with the Royal Yachting Association to clarify the requirements for the stowage of inflatable life rafts on such vessels." Kay Coombes, the sister of sailor Steve Warren, said: "We hope the safety recommendations are implemented by the sailing industry to help prevent a similar tragedy in the future. "We now await the findings of the Maritime Coastal Agency investigation, which we hope will provide further clarity regarding the incident. "This is a very difficult time for all the families involved, especially as we approach the first anniversary of the incident."