FILES expected to shed light on the controversial business dealings of Margaret Thatcher's son are to remain sealed for decades, leading to criticism from a former Labour MP who spent years searching for the truth.

The Downing Street documents relating to the activities of Sir Mark Thatcher had been due for release today under the National Archives rule that sees files eventually made public, currently after between 20 and 30 years.

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However, the release of a series of files related to Sir Mark, who allegedly made millions from a UK arms deal with Saudi Arabia and other projects while trading on his mother's name in the Middle East, will be kept secret.

Tam Dalyell, an MP between 1962 and 2005 who spent years voicing suspicion over the activities, said: "If information is withheld after all this time, one has to ask to whose advantage is it for it not to be published.

"My recollection is that this was a deeply unsatisfactory undercover relationship and it seemed at the time that he was taking advantage of his mother’s position. This not only troubled Labour politicians but a number of Tories as well. Again, I ask, to whose advantage is it that this information is suppressed for so long?"

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.

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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 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."

The former Prime Minister's son also caused embarrassment in 1982 when he went missing in the Sahara Desert while taking part in an international motor race. He was found, following a costly search and rescue mission which involved military aircraft, after six days and later admitted he had done no preparation.

In 2005, Sir Mark pleaded guilty to involvement in an alleged coup plot in Equatorial Guinea and was fined the equivalent of £265,000 and given a four-year suspended jail term by a South African court.

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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.