The gulf of Mexico oil spill may be many times larger than realised, with vast amounts of oil spilling out in deep undersea "plumes" which cannot be seen from the surface, scientists suggested yesterday, in a development which put yet more pressure on Britain's oil giant BP to cap the leaking well as soon as possible.

But in a first glimmer of positive news since the accident, BP reported last night that it had began successfully to extract both oil and gas to a ship via a more than mile-long tube inserted into the mouth of the leak on the seabed. An earlier attempt on Saturday to bring oil to the surface had to be suspended after the pipe became snagged.

Kent Wells, a BP vice president in Houston, refused to say exactly how much oil was being captured by the tube or put any kind of figure on the proportion of the leak that was being tamed. He said that measurements were being taken and enginners will know in the next day or two. But he added: "It will take a little time. We will look to capture as much oil as we can."

But as BP struggled to cap the well. the company's troubles increased with the news that the flow rate from the ruptured Deepwater Horizon drilling rig, which the US government calculated from satellite imagery to be about 5,000 barrels of oil a day, and which BP has gone along with, may actually be between 25,000 and 80,000 barrels per day (bpd), American researchers now believe (one tonne of crude equals 7.33 barrels, or 308 gallons.

The oil escaping from the rig, which BP had leased from the Swiss-based drilling company Transocean and which blew out on 20 April, in an explosion which killed 11 people, is already threatening an environmental and economic catastrophe along the Gulf coast.

But it could be much worse than so far anticipated. If the oil were flowing at the 80,000 bpd top rate now envisaged, unseen beneath the giant oil slick on the surface, the amount which has escaped since the accident would be more than two million barrels in total, which would make it the fourth worst marine oil-spill in history.

If the well cannot be capped from above and the leak can be stopped only by drilling a relief well, which has already begun but will take three months, the amount of oil leaked might amount to more than five million barrels. In June 1979, 3.5 million barrels escaped from the Mexican Ixtoc 1 well, also in the Gulf, after a blowout. Ixtoc 1 took nine months to cap.

The biggest oil-spill was deliberate, when valves at the Sea Island oil terminal in Kuwait were opened by Iraqi forces at the end of the Gulf War in January 1991, which is thought to have released more than 10 million barrels of oil into the Persian Gulf.

The new leakage rate assessment from Deepwater Horizon has come from scientists from several universities on board the US research vessel Pelican, on one of the first missions to gather scientific data about what is really happening in the Gulf, which has been backed by the National Oceanic and Atmospheric Administation (NOAA), the US agency which looks after the health of the seas. Their key finding has been that there are enormous plumes of oil hanging in the deep water, one of which is 10 miles long, three miles wide and up to 300ft thick.

"There's a shocking amount of oil in the deep water, relative to what you see in the surface water," said Dr Mandy Joye, of the Department of Marine Science at the University of Georgia, who is co-ordinating the mission from her laboratory in Athens, Georgia. "There's a tremendous amount of oil in multiple layers, three or four or five layers deep in the water column."

The shallowest oil plume the group detected was at 2,300ft and the deepest was near the sea-floor at 4,200ft. The scientists are alarmed that the underwater plumes are depleting the dissolved oxygen in the seawater. The oxygen had already dropped by 30 per cent near some of the plumes, said Dr Joye.

"If you keep those kinds of rates up, you could draw the oxygen down to very low levels that are dangerous to animals in a couple of months. It could take years, possibly decades, for the system to recover from an infusion of this quantity of oil and gas. We've never seen anything like this before. It's impossible to fathom the impact."

BP, the largest oil and gas producer in the US, which under US law is responsible for tackling the disaster and the subsequent clean-up, is coming under ever-increasing pressure over the length of time it is taking to stop the oil flowing, now nearly a month.