Anadarko Petroleum is expected to reveal it has been billed about $1bn (£630m) by BP so far for its share of cleaning up the Gulf of Mexico oil spill, when it reports results tomorrow.
The Texas-based company owns 25% of the Macondo well, but has so far refused to meet its share of the costs.
BP is in talks with Anadarko, and its third partner in the project, the Japanese firm Mitsui, which owns a 10% stake, over the issue.
Mitsui revealed that it had been billed $480m so far and said that it would not pay until investigations into the causes of the disaster are concluded.
If BP is found to have been grossly negligent – a charge which its own internal investigation has already cleared it of – it would be liable for all the costs. As the lead operator on the project, BP has said it would pay all the costs and seek reimbursement from its partners.
The Guardian has also learned that BP has only billed Anadarko and Mitsui for May and June. Bills for July are likely to be much bigger, as BP's operation to plug the leaking well reached its climax.
BP's outgoing chief executive, Tony Hayward, has indicated that the company will take legal action against its partners if necessary to recover the costs. BP last updated the cost of the spill on 19 July, when it amounted to $3.95bn.
Four official US investigations are under way into the causes of the explosion on the Deepwater Horizon rig, which killed 11 rig workers and resulted in the worst oil spill in US history.
BP announced yesterday it had sold its Colombian assets to Canada's Talisman Energy and Colombia's state-run firm Ecopetrol for $1.9bn. It is the latest in a string of asset sales as its tries to raise $30bn to meet its liabilities.
Last week the oil company announced it had set aside $32bn, assuming it will not be found to have been grossly negligent.
A new estimate of the size of the spill – 4.9m barrels – means the potential fine BP faces under US law could be between $5.4bn and $21bn, calculated at $1,100 (£691) a barrel – or $4,300 if the spill is deemed the result of gross negligence.