BP can no longer call the administrator of its $20bn (£12.5bn) Gulf spill claims fund "independent", a US federal judge ruled last night, which could hamper the company's attempts to cap its liabilities following the Deepwater Horizon disaster.

Judge Carl Barbier, of the federal court of Louisiana, said that BP must spell out in all future communications that the claims fund and its administrator, Kenneth Feinberg, are acting on behalf of BP in fulfilling its legal obligations under the Oil Pollution Act.

One US lawyer warned that the ruling could invalidate the promise made by 87,000 claimants who have received a lump sum payout from the fund that they would not seek further damages from BP in the courts.

Feinberg, who was appointed last summer by the White House to take over the claims process, has advised individuals and businesses seeking compensation that they do not need their own lawyer to agree a payout. In television interviews and town hall meetings across the Gulf, he has repeatedly said that claimants would be better off receiving a lump sum payment from the fund rather than seeking compensation through uncertain and lengthy legal action.

He, the US government and BP have all described the fund he administers, the Gulf Coast Claims Facility (GCCF), as "neutral" and "independent".

But in a 15-page ruling, Judge Barbier said: "While BP may have delegated to Mr Feinberg and the GCCF independence in the evaluation and payment of individual claims, many other facts support a finding that the GCCF and Mr Feinberg are not completely 'neutral' or independent from BP."

BP also is now obliged to tell claimants that they have the right to consult a lawyer and add their names to the many US lawsuits being prepared to seek damages independently of the fund.

One attorney representing spill claimants said that the ruling could result in the 87,000 settlements being re-examined.

Kevin Dean of Motley Rice in South Carolina told Bloomberg: "It's not a can of worms. It's a 55-gallon drum of worms." He added his firm would start to contact clients who had accepted settlements from the claims fund to "tell them of their rights".