Optimism about the planned "static kill" which will involve the injection of drilling mud into the top of the well, helped fuel a rally in BP and other energy shares on August 2 despite reports that US regulators are investigating insider trading in the British energy giant's stock.

But BP said a test planned for August 2  ahead of the operation would now likely take place the following day because a hydraulic leak was detected. BP had been aiming to begin the static kill itself on August 3  and remained cautiously optimistic it could still do so.

The well was temporarily sealed in mid-July but the static kill followed by the completion of a relief well later in August are seen as the permanent solutions to the leaking well, which US government scientists estimate has released almost 5 million barrels of oil since late April.

The worst offshore oil spill in U.S. history has been an environmental and economic nightmare for the Gulf coast.