Heritage in 2009 agreed to sell its Ugandan holdings to Italian energy company Eni. Tullow had the right to pre-empt the deal because of existing interest and in early 2010 the Ugandan government said it backed Tullow's plans despite a lobbying campaign by Eni to stop the transaction.
Tullow in April 2011 filed a claim in a British court claiming Heritage violated its sales contract by not reimbursing it for tax payments made to the Ugandan government.
Tullow said the Ugandan government had ruled Heritage should pay $313 million to Tullow's subsidiary as part of the transaction and the high court agreed.
"A further hearing will be scheduled in due course to address matters arising from the judgment such as the amount of interest Heritage owes Tullow on the $313 million and Tullow's submissions in respect of payment of its costs by Heritage," the company said in a Friday statement.
Heritage said in a statement it "strongly disagrees" with the decision.
"Heritage will perform a robust and exhaustive evaluation of its legal options with a view to appealing the judgment," it said Friday.