Diana died in a car crash in Paris, France in 1997 along with her lover Dodi Fayed and chauffeur Henri Paul. The couple's bodyguard survived.
The five-year investigation into the tragedy found Paul, 41, was over the country's drink-drive limit and ruled the deaths accidental.
However, in a subsequent examination by France's civil Court of Grand Instance, a judge found some aspects of the investigation "dysfunctional," including a mix-up over Paul's post-mortem and blood tests which delayed the inquiry for two years.
The judge criticised French authorities for failing to respond quickly enough to a complaint by Fayed's father Mohamed Al-Fayed, who alleged Paul's blood tests had been forged to make him appear drunk behind the wheel. Subsequent re-testing delayed the investigation.
The French judge maintained that although "formal mistakes" had been made, they "had no long-term repercussions on the revealing of the truth."
The court ordered the state of France to pay $7,500 (£5,000) in damages to Al-Fayed.