The top United Nations court will on Monday rule on a decades-long dispute between Gabon and Equatorial Guinea over three islands in potentially oil-rich waters.

The two countries have been squabbling over the 30-hectare (74-acre) island of Mbanie and two smaller low-lying islets, Cocotier and Conga, since the early 1970s.

The islands themselves are tiny and virtually uninhabited but lie in an area potentially rich in oil and gas.

The dispute dates all the way back to 1900, when then colonial powers France and Spain signed a treaty in Paris setting out the borders between the two countries.

But Gabon believes that a later treaty, the 1974 Bata Convention, then fixed the islands' sovereignty in their favour.

‘Scraps of paper’

The Bata Convention "resolves all sovereignty issues regarding the islands and border delimitation", Marie-Madeleine Mborantsuo, honorary president of Gabon's constitutional court, told the International Court of Justice (ICJ) in hearings in October.

Equatorial Guinea argues that Gabon invaded the islands in 1972 and has occupied them illegally ever since.

Lawyers for Equatorial Guinea rubbished the Bata Convention in the October hearings, saying Gabon suddenly produced the document in 2003, surprising everyone.

"No one had seen or heard of this supposed convention," Domingo Mba Esono, Vice-Minister of Mines and Hydrocarbons from Equatorial Guinea, told ICJ judges.

"Moreover, the document presented was not an original but was only an unauthenticated photocopy," said Esono.

Philippe Sands, a lawyer representing Equatorial Guinea, dismissed the Bata Convention as "scraps of paper".

‘Archives badly managed’

"You are being asked to rule that a state can rely on a photocopy of a photocopy of a purported document, the original of which cannot be found and of which no mention was made or any reliance placed for three decades," said Sands.

Equatorial Guinea has been asking for an original copy of the Bata Convention since 2003, so far in vain.

Mborantsuo admitted that "sadly, neither of the two parties can find the original document", noting it was drawn up in an era before computers and databases.

"Archives were badly managed because of a number of things -- unfavourable climate, a lack of trained personnel and lack of technology," said Mborantsuo.

Unlike most countries appearing before the ICJ in The Hague, which rules in disputes between states, Guinea and Equatorial Guinea agreed to ask judges for a ruling in an effort to find an amicable solution.

The two countries have asked the court to decide which legal texts are valid -- the Paris Treaty of 1900 or the Bata Convention of 1974.

The ICJ will not rule which country should be granted sovereignty over the islands.

"We are convinced the court's judgement will help our countries resolve their outstanding disputes over sovereignty and borders, creating a sustainable basis for their relations to flourish," said Equatorial Guinea's Esono.