Downing Street files relating to the controversial business dealings of Margaret Thatcher's son - Sir Mark Thatcher - are to remain closed to the public for years to come, it has emerged.
Under public records legislation, official files should be released to the National Archives in Kew, west London, after 20 years unless there are specific reasons for them not to be disclosed.
However, the latest release of files to the National Archives includes two files entitled "Cementation contract: Mark Thatcher and the Omanis" covering the period from 1981 to 1988 which are listed as being retained for 65 years - meaning they will not be released until 2053.
Two other files - one entitled "Mark Thatcher and the Omanis; other allegations against Mark Thatcher" and "Request by Electronic Data Systems to employ Mark Thatcher" have been marked "temporarily retained" with no date for release.
The Cementation contract refers to a deal by Cementation International - a subsidiary of the Trafalgar House property, construction and engineering conglomerate for which Sir Mark was working as a consultant - to build a new university in Oman.
When details emerged in the press in 1984, it led to allegations that Mrs Thatcher had used her influence with the pro-British Sultan Qaboos of Oman to secure the contract for her son's firm.
In the ensuing furore it was decided that Sir Mark should leave the UK and move to live in the United States.
In the latest volume of his authorised biography of Mrs Thatcher, Charles Moore quotes her private secretary at the time, Robin (now Lord) Butler, as saying he thought her conduct in relation to the deal "conveyed a whiff of corruption".
He told Mr Moore: "She had wanted to see Mark right. She sought the deal for Mark. She excluded everyone from her talks with the Sultan.
"Mark was dealing with Brigadier Tim Landon who was the Sultan's go-between. She behaved in a most peculiar way. I suspected the worst."
Among the other files listed as being retained is a series of No 10 papers about the royal family, including one entitled "Career of Prince Andrew Duke of York", another entitled "The Prince of Wales' Special Projects Unit" and a third marked "Procedures for the Royal Family taking up jobs and other commercial appointments".
Also retained is a series of files relating to the Spycatcher case, which concerned the Government's efforts to suppress the memoirs of former MI5 officer Peter Wright.
The Cabinet Office said a "small number" of records had been retained because they contained "personal data about individuals and sensitive information relating to other countries".
"This is in line with the requirements of the Public Records Act," a spokesman said.
The decision to retain the files was condemned by Liberal Democrat leader Tim Farron, who urged the Cabinet Office to think again.
"Sunlight is the best disinfectant and this decision is ludicrous. If certain sections still compromise national security or foreign relations after all this time, they should be redacted," he said.
"Given the topic, it seems more likely that this is a move to avoid embarrassment. There is no legitimate reason for these papers to not be in the public domain and I urge the Cabinet Office to think again."