The compensation has been claimed under the settlement that BP agreed in 2012 for businesses and individuals affected by the spill, which has turned out to be significantly more costly than the company had projected.
Patrick Juneau, the compensation administrator, allowed businesses to present calculations for the losses they had suffered as a result of the spill on more generous terms than BP had expected, making the company liable for much bigger payouts. The company's attempts to fight those payments through the courts met with only partial success.
The deadline for businesses to file compensation claims was June 8, and by June 1 more than 115,000 forms had been submitted.
Of those, about 18,000 have been paid, and about 32,000 denied, withdrawn or otherwise closed, leaving a potential backlog of more than 60,000 going through various stages of processing, including awards that have been appealed.
There is likely to have been a surge of claims in the final week as businesses rushed to beat the deadline.
In the stock exchange announcement for its first-quarter earnings, BP said there was uncertainty over the final number of claims and over the expected average payout per claim, because "determinations to date may not be representative of the total population of claims".
BP's principal victory in the legal battles over the settlement is that businesses are now forced to "match" their revenue against corresponding expenses when making calculations of how much they lost as a result of the spill.
That new standard used by Mr Juneau's team, known as Policy 495, appears to have reduced the average size of awards for business losses since it was adopted last May.
Payouts since then have averaged about $96,000 per claim, compared to $209,000 per claim under the previous, less restrictive, policy, according to Financial Times calculations based on data published by Mr Juneau's office.
However, even using that lower figure, the remaining unprocessed claims for business losses faced by BP could take the final settlement total to more than $2 billion above the $10.3 billion it has provided for.
BP declined to comment on the calculations by the Financial Times.
The Deepwater Horizon Claims Center led by Mr Juneau has already paid out $5.2bn under the settlement. BP's original forecast of the cost was $7.8 billion.
Brian Gilvary, BP's chief financial officer, said on a call with analysts last week that the company would give an update on the costs of the settlement with its second-quarter results on July 28, but a comprehensive estimate of the liability would not be available until the third quarter or more likely the fourth-quarter earnings.
As well as remaining claims under the settlement, BP still faces other claims from businesses and individuals in the Gulf of Mexico region, including those that chose to opt out of the settlement, financial institutions, property developers, and those in the oil and gas industry who lost earnings when the US government banned drilling in the gulf for six months.