Some 30 tankers remain anchored a few miles off Malta, months after experts said companies had bought oil when it was at its lowest price and stored it at sea.

Back in June, Reuters had said about 20 million barrels of oil, around a quarter of the world's daily demand or enough to supply the island for three years, were anchored at sea just outside Malta's territorial waters.

According to Sullivan Maritime general manager Joe Bugeja, it was a well known fact in the industry that there were oil products stored at sea outside Malta.

Not all the tankers are laden with oil products. "There are vessels awaiting orders, others waiting for a crew change, others for the price of oil to go up," he told The Times. He said it was difficult to say how long the tankers would remain off Malta. "It depends on how much they bought the oil for, how long they are willing to wait and a number of other factors," he said.

Other industry sources said it was cheaper for shipping companies to leave the tankers at sea, outside territorial waters where they were not paying anchorage charges. "It is much cheaper to leave them out there," the sources said.

Other tankers are waiting for their next assignment since the global recession had badly hit the shipping industry.

In fact, lower demand for shipping pushed down the price of leasing tankers, which came as an added bonus for companies which bought stocks of oil when the prices were low and were storing them at sea until the prices went up.

The price of oil has indeed risen recently but, at under $80 a barrel, it is still far from the almost $150 high it reached last year.

The lack of demand for oil is one reason why the tankers may not be unloading their stocks, industry sources said. Another scenario could relate to the futures markets, contracts that oblige a buyer and seller to exchange a product at a future date at a prearranged price. The lower costs of leasing tankers could make it more attractive for oil to be stored at sea until the contract date comes round.

In June, Reuters reported that American investment bank JPMorgan Chase and Co. had hired a tanker to store gas oil off Malta's coast.

The tankers are at Hurd's Bank, some 18 miles off Malta, and fears had been expressed that they could pose a major environmental threat to the island in the event of a leakage. But the Malta Maritime Authority had given a reassurance that the anchored vessels did not pose a greater or lesser risk than normal daily traffic.