Classified US cables from WikiLeaks will be used to cross examine senior UK government officials, after a judge ordered the unprecedented step.
Although lawyers acting for Foreign Secretary William Hague objected, a High Court judge ruled on Wednesday the move was necessary to resolve "fairly and justly" a claim launched against the Government by exiled residents of the Chagos Islands.
Justice Stanley Burton declared the leaked US cable - alleged to relate to a private US-UK diplomatic meeting - could be investigated in court even though it must have been obtained unlawfully by "the notorious internet organisation".
The British expelled the Chagos islanders between 1965 and 1973 in order to allow the US to establish an airbase on Diego Garcia, the largest island in the Chagos archipelago.
The islanders have been fighting a long campaign, which has included a string of court cases, for the right to return to at least some of the islands.
Wednesday's hearing relates to a new High Court application by the Chagos Refugees Group for judicial review of the Government's decision to prevent commercial fishing on the island by making it a Marine Protected Area, which the islanders say was a ruse to stop them from returning.
They say their case is supported by a cable obtained by WikiLeaks, sent by the US embassy in London to the US State Department in Washington in May 2009.
Colin Roberts, commissioner for the British Indian Ocean Territory, will be cross-examined about the content of the cables.
Roberts is reported in the cable to have asserted "establishing a marine park would, in effect, put paid to resettlement claims of the (Chagos) archipelago's former residents."
He also applied for permission to cross-examine Joanne Yeadon, a civil servant at the Foreign and Commonwealth Office at the talks with the Americans.
The judge said: "I understand why it is the policy of HM Government neither to confirm nor deny the genuineness of leaked documents, save in exceptional circumstances, particularly where, as here, the documents in question are not those produced or received by the UK Government.
"However, the documents have been leaked and indeed widely published.
He ruled: "I do not see how the present claim can be fairly or justly determined without resolving the allegation made by the claimant, based on the WikiLeaks documents."