SYDNEY, Australia — A Chinese company’s agreement to lease an entire island in the South Pacific has been ruled illegal by the attorney general of the Solomon Islands, reversing or at least delaying a major push into a location long prized for its strategic value.

The deal for the island, Tulagi — a headquarters for world powers both before and during World War II — granted exclusive development rights for at least 75 years. It was signed by the head of the provincial government and executives from China Sam, a Beijing-based conglomerate founded as a state-owned enterprise.

The attorney general’s office said in a statement on Thursday that the agreement should be terminated because it lacked vital details, such as timelines, and encroached on the powers of the national government.

The statement noted that China Sam had not registered as a foreign investor, and yet tried to secure the right to negotiate with every other Chinese company that might be interested in Tulagi.