BP prepared on Monday to deliver the first of two blows it hopes will "kill" its oil well in the Gulf of Mexico, 105 days since the start of a leak that led to the U.S. environmental disaster.

The White House said it was "cautiously monitoring" the situation as BP engineers, mindful of previous setbacks to permanently seal the well, conducted final tests.

The tests will show whether BP can move ahead on Tuesday with its planned "static kill" - when it will inject heavy drilling mud into the top of the well to push oil back where it came from: a reservoir 4,000 meters beneath the seabed.

"When we've done that ... we will have killed the well," said Kent Wells, BP's senior vice president of exploration and production.

To ensure the ruptured well is permanently sealed, BP will also proceed with its plans for a relief well that will intersect the damaged well and pump more heavy drilling mud and cement in from below to plug any openings.

Wells said the British company expected the relief well to intercept its blown-out Macondo well between Aug. 11-15.

A permanent seal to the well will end the months-long technical nightmare for BP that began April 20 with an explosion on a rig 80 km off the Louisiana coast.

Millions of liters of crude spewed into the ocean after the blast, which killed 11 workers and ruptured the well a mile beneath the surface of the sea.

But even when the well is finally capped, the environmental disaster will be far from over and the political and financial implications will reverberate for some time to come.

Gulf fishing communities and business owners are still counting the costs of the worst offshore oil spill in U.S. history that forced the closure of large swaths of rich fishing grounds and dealt a severe blow to tourism.

BP managed to put a tight-fitting cap on the well two weeks ago that temporarily stopped the flow of oil into the ocean.

Cautious Optimism
But after months of uncertainty, BP investors are looking for a permanent solution to disaster that has cost the company billions of dollars in cleanup costs, tarnished its reputation in the U.S. and left it facing numerous lawsuits and possible criminal charges.

BP, which has lost about 40% of its market value since the start of the crisis, saw its share price rise 1.8% in London. U.S. listed shares were up nearly 2% in midday trading in New York.

"The end is in sight," said Peter Hitchens, an oil analyst at Panmure Gordon.

If BP's two-pronged effort to cap the well is successful, it would also be good news for President Barack Obama, who has seen his public approval rating slip in recent months, partly because of voter unhappiness over his handling of the spill.

BP's response to the spill also sparked widespread public anger in the U.S. and last week the company announced its chief executive, Tony Hayward, was stepping down. He had been under pressure after a series of verbal gaffes.

Hayward's replacement, Bob Dudley, will fly to Moscow this week to meet government officials and partners in the oil company's Russian venture - his first visit since fleeing the country in 2008.

A BP spokesman said the visit was to allow Hayward to "re-introduce" Dudley to Russian shareholders and to Russia's top energy official, Deputy Prime Minister Igor Sechin.

When he left Russia in 2008, Dudley said he had been forced out under a campaign of harassment by billionaire partners in the TNK-BP joint venture of which he was CEO.

BP has since settled its dispute about control of TNK-BP and the venture remains a key part of BP's portfolio.