BP admitted it is capturing less oil than initially estimated from the broken well in the Gulf of Mexico yesterday, and the group is ramping up efforts to try yet another scheme to plug the leak.

The mile-long pipe siphoning oil and gas from the broken well for the past eight days is the first successful effort to contain the leak conservatively estimated to be pumping out 5,000 barrels of oil per day. But since the so-called "riser insertion tube tool" (RITT) went into operation it has collected an average of just 1,885 barrels per day, according to the latest statistics – far from the 3,000 barrels estimated last week.

BP says that the amount of escaping hydrocarbons captured by the RITT and shipped to the surface has fluctuated from 1,120 to 3,000 barrels per day because of the erratic behaviour of the well.

Tomorrow BP will move on to its fifth attempt to stem the spill. The "top kill" operation will pump heavy fluids into the broken well to push down the gas and oil, with a view to sealing the well permanently with a concrete cap (see box, right).

So far the spill has cost BP $760m (£529m), including the response, federal costs, "hardship claims" from businesses and individuals immediately affected and grants to Gulf states. More than 1,100 vessels are involved in trying to control the spill, some 243,000 barrels of oily liquid have been skimmed from the surface, and nearly 2.5 million ft of floating boom have been deployed.

The fatal explosion on Trans-ocean's Deepwater Horizon rig on 20 April has turned a spotlight on safety standards in the US oil industry. Washington has stressed repeatedly that BP will be financially liable for the full costs of sealing the well and cleaning up the slick, which has already polluted 65 miles of the Louisiana coast.

Ken Salazar, the US Secretary of the Interior, upped the pressure at the weekend, claiming he is "not completely" confident in BP and that the company has missed "deadline after deadline". "If we find out they're not doing what they're supposed to be doing, we'll push them out of the way appropriately," Mr Salazar said.

BP said it was part of a multi-agency team involving several other oil companies, all of whom were doing all they could to solve the problem. It also yesterday announced a $500m research programme to study the impact of the incident, and the response, on the environment.

Green groups are concerned that the massive use of dispersants to break up the slick may be causing as much damage as the oil.

Industry insiders yesterday dismissed Mr Salazar's comments at politically motivated. "BP is doing as well as anyone would have, and everybody is collaborating to help," one source said.

But investors are increasingly spooked at the open-ended costs BP faces, and the enormous damage to its already fragile reputation in the US. The group's share price dropped by another 2.7 per cent yesterday to 493p, adding up to a collapse of nearly 25 per cent in the month since the explosion, wiping around £30bn off the company's market capitalisation.

This week's operation will be crucial, says Malcolm Graham-Wood, at Westhouse Securities. "The top kill is pivotal to what happens next," he said. "If it's a success then the whole thing will become a clean-up operation with clear, albeit high, costs."

Several City analysts have consistently held that the sell-off of BP's stock has been overdone. But if the top kill does not work, the uncertainties facing the company will become a genuine threat. And with the hurricane season due to start in a couple of weeks, the dangers are multiplying.

"All the bull arguments about BP's long-term position still stack up at the moment," Mr Graham-Wood said. "But a lot of people are starting to say there are other stocks they would rather be in, and if the top kill doesn't work then the downside risks start to become more significant."

Tried and testing: BP's efforts so far

*Tomorrow, BP will attempt "top kill", squirting "mud" – the slurry used to cool, lubricate and control the pressure of drilling operations – into the well to suppress the leak and allow it to be capped with concrete. It may also attempt a "junk shot" – firing pelletised rubber into the well's network of pipes to seal any small holes and ensure the mud pushes the oil and gas back down into the reservoir.

*The riser insertion tube tool (RITT) put in place last week was the first success. A proportion of the spill is now being sucked up to a ship where the oil is stored and the gas flared off.

*BP is investigating whether the failed "coffer dam" – a large drum placed over the leak so it can be siphoned off – could be more successful now that some oil is being removed by the RITT. Another possibility is to put a second blow-out preventer (BOP) on top of the failed one.

*Two new rigs are already in place and are drilling relief wells to divert the flow. The plan is guaranteed to work, but such wells take 90 days to drill.

*The first attempt at the coffer dam failed because hydrates formed by the gas reacting with the cold sea water blocked the container before it was even fully lowered over the leak.

*A smaller "top hat", less prone to blockage, is on the sea floor nearby but has not been used because of the subsequent success of the RITT.