* BP shares fall 8 pct
* Fears oil spill larger than thought
* Obama lays responsibility squarely on BP
* BP prepares to drill relief well
By Tom Bergin
LONDON, May 3 (Reuters) - Shares in oil major BP Plc
fell further on Monday, as fears grew the amount of oil gushing from one of its wells in the Gulf of Mexico, and the cost of cleaning it up, may be far greater than previously thought.
UK markets were closed on Monday but BP's Frankfurt-listed shares opened 8 percent lower before recovering to trade down 3.8 percent at 1008 GMT, against a 0.3 percent fall in the STOXX Europe 600 oil and gas index
The losses are on top of a drop of around 13 percent since the accident, equating to a drop of around $20 billion in BP's market capitalisation.
"The problem with BP is, as long as the leak continues, nobody knows how much it'll cost to clean up the mess, so the stock could continue to sink for a while," one Paris-based trader said.
President Barack Obama visited Louisiana at the weekend and laid the cost of the clean up firmly on BP.
BP has not put an estimate on the likely costs of the spill, which follows the explosion and sinking of a drilling rig operated by Swiss-based driller Transocean
The company said last week it and partners in the well, including Anadarko Petroleum
, were paying $6 million/day in clean-up efforts but admitted costs would rise sharply when the oil hit land, as would damages claims.
The final cost will be determined by the quantity of oil spilt and the amount that hits the shore.
Last week, initial estimates of the leak from the Macondo well were raised to 5,000 barrels per day from 1,000 bbls/day. Over the weekend, BP said it was impossible to say how much oil was being spilt.
Some media reports cited experts as saying aerial pictures suggested as much as 25,000 barrels/day could be flowing, while Ken Salazar, U.S. Interior Secretary, said as much as 100,000 bbls/day could flow in a worst-case scenario.
Based on the 5,000 bbls/day flow, Neil McMahon, analyst at brokerage Bernstein, estimated the clean-up costs could hit $7 billion, although analysts at Morgan Stanley estimated the costs at $3.5 billion.
Damages to fisheries and the tourist trade could be $5.5 billion, Bernstein estimated.
BP said in a statement on Monday it was committed to promptly pay legitimate claims for clean-up, property damage and personal injury caused by the spill, as well as commercial losses, including loss of earnings, and other losses.
BP is unlikely to be able to reclaim any damages from the drilling contractor, Transocean, analysts at Morgan Stanley said. "(Transocean) likely is contractually protected from any environmental liability," the bank said in a research note.
At the weekend, BP was preparing to break ground on a new well that aims to shut off the leaking well. (Additional reporting by Blaise Robinson; Editing by David Holmes)