The £3.9bn ($6.8bn) sale of UK ports and shipping group P&O to Dubai Ports World has been given the go ahead by the Court of Appeal.
Buying P&O would turn DPW into the world's third-biggest port operator
The court on Monday rejected an attempt to block the deal by Miami-based Eller, a business partner of P&O in the US.
Eller had claimed that concerns about a United Arab Emirates-based company buying P&O's operations at six major US ports would harm its business.
The Court of Appeal has backed last week's approval by a High Court judge.
Eller said it was very disappointed at the Court of Appeal's decision.
"We are not alone in opposing this deal - many elected politicians in the US and indeed the American public, share many of our concerns about the takeover," it said.
Eller added that while it appeared to have exhausted its legal fight in the UK, it would continue to pursue legal action against the takeover in the US.
London-based P&O operates at 21 ports across the US, including Miami, New York and New Orleans.
Dubai Ports World (DPW) has already received regulatory approval in the US for the deal from the federal Committee for Foreign Investment in the United States.
The Dubai-based business has also volunteered to submit itself to a second 45-day investigation of the deal's potential security risks.
Eller's objection to the deal has been mirrored by both Republican and Democrat politicians in the US, but President George W Bush has repeatedly insisted the deal would not compromise national security.
Buying P&O will turn DPW into the world's third-largest port operator.