New Orleans, Louisiana (CNN) -- Federal authorities have ordered BP to get more aggressive with its plans to recover thousands of barrels of oil spewing from a broken well into the Gulf of Mexico, according to a letter made public Saturday.

In the letter, Rear Adm. James Watson, the government's on-scene incident manager, gave BP 48 hours to identify and expedite other ways to contain oil, given new estimates that doubled the amount of crude gushing out every day.

Researchers reported this week that up to 40,000 barrels, or 1.7 million gallons, a day may have escaped from the BP well in the 54 days since the explosion aboard the Deepwater Horizon drilling platform in April.

That would mean almost 100 million gallons have spewed since the rig exploded -- many times the amount spilled by the supertanker Exxon Valdez in Alaska's Prince William Sound in 1989.

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"It is clear that additional capacity is urgently needed," Watson said in the letter Friday to Doug Suttles, BP's chief operating officer.

"I am concerned that your current plans do not provide for maximum mobilization of resources to provide the needed collection capacity consistent with revised flow rates," Watson said. "I am also concerned that your plan does not go far enough to mobilize redundant resources in the event of an equipment failure with one of the vessels or some other unforeseen problem."

BP has captured some of the gushing oil through a containment cap that has been pumping the crude up to a drilling ship for about a week. It collected 15,500 barrels, or 651,000 gallons, on Friday and has been able to remove a total of 104,300 barrels, or about 4.4 million gallons, from Gulf waters so far.

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BP spokesman Mark Proegler said company officials are in the process of reviewing Watson's letter.

"We will be responding directly to him regarding containment plans and will work directly with the Coast Guard to keep the public informed of any changes to the process," Proegler said in a statement.

Suttles, who was in Houma, Louisiana, on Saturday meeting with BP workers and Coast Guard officials, said Watson can expect a response Sunday.

"We're looking at everything we have in place and we're looking to see if there's anything more we can do, and we will respond by tomorrow night," he said.

The government asked BP to lay out plans for additional and contingency recovery methods in a letter sent Tuesday. BP responded Wednesday by describing a three-element plan that included the current container cap, a choke line to pump additional oil to the surface and a kill line intended to capture oil left in excess after the first two methods.

The kill line, said Suttles in a letter to Watson, would not be operational until mid-July, a date that was unacceptable to the government.

"You indicate that some of the systems you have planned to deploy may take a month or more to bring online," Watson said. "Recognizing the complexity of this challenge, every effort must be expended to speed up the process."

BP's orders to step up came as oil kept gushing and frustration kept flaring.

President Obama, who intends to return to the Gulf next week, discussed the oil disaster Saturday with British Prime Minister David Cameron, according to a statement from the prime minister's office.

A diplomatic spat of sorts had erupted over perceptions that Obama was attacking Britain with his tough words for BP. Obama sought to ease the tension by stressing the importance of U.S. trade ties across the Atlantic.

In his telephone conversation with Cameron, Obama said "frustrations about the oil spill had nothing to do with national identity," according to a statement from Downing Street.

Cameron had spoken Friday with BP Chairman Carl-Henric Svanberg, making it clear that the oil giant's financial health and stability were in Britain's best interests.

Svanberg, who is to meet Wednesday with Obama at the White House, "made clear that BP will continue to do all that it can to stop the oil spill, clean up the damage and meet all legitimate claims for compensation," a Downing Street spokesman said in a news release.

Meanwhile, environmentalists said demonstrations were unfolding Saturday in more than 50 cities across five continents -- from Pensacola, Florida, to Christchurch, New Zealand.

"Let the world know YOU care," said a flier on the group's Facebook page, which translated BP's initials to mean British Predator. "We need to let BP know that we are NOT okay with what they are putting in OUR oceans."

About a dozen protesters gathered under a BP gas station sign in Atlanta, Georgia, picketing and cheering as motorists who drove by honked.

"I don't ever intend to use BP again," said Atlanta resident Monica Manuel who held up a cardboard sign, "Boycott big polluters."

Another protester, Ruth Resnicow, said it was devastating to see the wildlife on the Gulf Coast being affected by the worst oil disaster in U.S. history. "The oiled pelican has been a logo of the disaster. People need to see change."

But gas station and convenience store owners said protesters are targeting the wrong people.

"The hardest part is on the employees because they are seeing the protests. ... They are concerned about their jobs and families," said Russell Scaramella, who owns 22 BP stations in the Atlanta area.

Jeff Lenard, a spokesman for the National Association of Convenience Stores, said Saturday's protests were likely to be counterproductive.

"So whether you are protesting by marching, or by not buying gas, you are hurting the small business," he said, "but you are really not hurting BP."