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The intelligence documents are believed to implicate a militant group in the deadly of Pan Am Flight 103.

Lawyers for the family of Abdelbaset al-Megrahi, the convicted in the bombing, believed the documents were central to their appeal case beginning on Tuesday.

The appeal was lodged by Megrahi's son, Ali Abdulbaset al-Megrahi, and is believed to be the first posthumous miscarriage of justice case in Scottish history. Megrahi died of cancer in 2012.

The new appeal hearing was ordered after the Scottish Criminal Cases Review Commission found there were significant discrepancies in the evidence given by the prosecution's key witness. 

The comission also found the Crown had failed to disclose the key witness, Maltese shopkeeper Tony Gauci, and his brother were offered £3 million ($4 million) reward payments for testifying and therefore that Megrahi's right to a fair trial had been breached.

The intelligence documents were reportedly sent by King Hussein of Jordan and are thought to allege that the bomb was made by a Jordanian intelligence agent inside the Popular Front for the Liberation of Palestine-General Command (PFLP-GC).

Critics of Megrahi's conviction argue that the fatal bombing was carried out by the PFLP-GC on behalf of Iran, in revenge for the shooting down of an Iranian passenger jet by a US warship that same year.

All 290 people aboard Iran Air Flight 655 were killed in the 1988 incident; likewise, all 259 passengers and crew aboard Pan Am Flight 103 were killed when a bomb exploded above Scotland. An additional 11 people were killed when parts of the plane crashed onto a residential street in Lockerbie.

Supporters of the PFLP-GC claim believe Libya was implicated in the bombing to cover up Iran's alleged role.

British Foreign Secretary Dominic Raab signed a public interest immunity certificate to keep the documents secret in August. Then-Foreign Secretary David Miliband also signed such an order in 2008.

Scotland's most senior judge, Lord Carloway, upheld the order in a ruling issued late on Friday.

Raab's "clear view is [it] would cause real harm to the national security of the UK because it would damage counter-terrorism liaison and intelligence gathering between the UK and other states," Carloway said, according to The Guardian.

"The documents had been provided in confidence to the government. Their disclosure would reduce the willingness of the state, which produced the documents, to confide information and to co-operate with the UK," Carloway continued.