Oil company BP says it has cut a ruptured pipe from the leaking Gulf of Mexico oil well, a key step in the latest effort to cap the well.

BP chief Tony Hayward hailed it as an "important milestone" and said BP could know within the next 12 to 24 hours if the capping effort would succeed.

For now, undersea robots are preparing the well-head to receive the cap.

President Barack Obama is to visit the gulf on Friday, his third trip there since the leak began six weeks ago.

The White House has been under increasing pressure to show the administration is in control of the response and clean-up efforts.

The federal government will send a $69m (£47.1m) bill to BP Plc on Thursday for expenditure incurred from its response to the spill, the White House said.

Meanwhile, a detailed computer modelling study indicated that oil from the spill might extend along thousands of miles of the Atlantic coast and open ocean as early as this summer.

The National Center for Atmospheric Research said once oil entered the "Loop Current", part of the Gulf Stream which sweeps around the Florida panhandle, it would be only weeks before it reached Florida's Atlantic shores.

Attempt to cap oil leak The latest stage in BP's efforts to contain leaking oil has involved lowering a cap onto the failed blowout preventer (BOP) valve system on the seabed. The cap sits on the BOP's lower marine riser package (LMRP) section. First, the damaged riser - the pipe which takes oil from the well - was cut where it nears the seabed using a remotely-operated shear. This was completed at 1930 CDT on 1 June (0030 GMT 2 June). The next stage was for a diamond wire cutter to saw through the riser close to the LMRP. The blade got stuck and had to be removed but BP eventually cut through the pipe using giant shears manipulated by undersea robots (ROV). After removing the pipe, the cap was lowered onto the LMRP enabling the leaking oil and gas to be funnelled to a drill ship on the surface. Latest estimates suggest more than half of the leaking oil is now being captured. BACK {current} of {total} NEXT

Speaking in Houston, Mr Hayward described the pipe-cutting procedure as "simply the beginning".

Giant shears

BP used giant shears manipulated by undersea robots to snip off the end of the pipe, after a diamond-edged saw failed to do the job.

The challenge is now to place a containment cap securely over the cut and stop the oil flow. The company hopes to collect the oil on a surface ship above the well.

"Our task is to contain the oil, ultimately to eliminate the leaking well, and most importantly, to clean up all of the oil, defend the shoreline and restore the shoreline where oil does come ashore such that we return it to its original state," Mr Hayward said.

BP estimates that the disaster has so far cost the company about $990m in clean-up costs.

Two Democratic senators have written to Mr Hayward urging him to suspend payments to shareholders worth $10bn until all costs of the clean-up and compensation are paid out.

On Wednesday, BP said it would pay for the construction of six sand barriers off the coast of the US state of Louisiana.

'Fair criticism'

The sand barrier project will push BP's bill to about $1.4bn.

BP share prices have continued to plummet in trading on the London Stock Exchange, amid news the US justice department has opened several civil and criminal inquiries into the Gulf spill.

BP IN NUMBERS Profits in 2009: $13.96bn (£8.75bn)

Clean-up costs so far: $990m (£674m)

Estimated cost of sand barrier project $360m (£244m)

Dividend payment 2009: $10.5bn (£7bn)

BP scheduled to make a first-quarter dividend payment to shareholders on 21 June

At its worst BP saw 34% wiped off the company's value

Share prices on London Stock Exchange fell from a pre-disaster high of 655.4 pence to 429.75p

Mr Obama has used the disaster to urge Congress to pass a bill to overhaul US energy policy and end tax breaks for oil companies.

The oil began leaking into the Gulf of Mexico on 20 April when the Deepwater Horizon drilling rig, leased to BP, exploded, killing 11 workers.

BP is drilling two relief wells to permanently stop the leak but they are not expected to be completed until August.

A "top kill" procedure, which had been considered the best hope for plugging the leak, failed over the weekend.

Engineers were unable to pump enough heavy mud into the well to staunch the oil flow.