In a statement issued to the Independent last night, he said he had no obligation to declare these interests and was "absolutely correct to act as I did". He said he received remuneration from only one of the five directorships but did not disclose any payments he received.
Records at Companies House show that he was director of a company which was directly funded by a member of the Saudi royal family. In another of the companies he was the majority shareholder, but his statement last night said the company had been dormant for 14 years.
He resigned his directorships after becoming Minister for Defence Procurement in 1992.
Mr Aitken's detailed statement came as he faced increasing pressure to carry through his threat to take legal action against Granada Television's World in Action, which on Monday night examined his links with with the Saudi royal family and prominent Saudi businessmen.
He made an unprecedented public statement on Monday in response to reports in the Guardian that morning. He denounced "wicked lies" in the newspaper and the programme and said he would sue the paper for libel. The Guardian said it stood by its story.
Mr Aitken said he would issue similar writs against the programme makers, but the statement yesterday said he was examining a transcript of the programme with his legal advisers. A further announcement would follow after Easter.
The statement added that he was considering legal action following recent articles in the Independent. These disclosed that he had been a director of a British arms company, BMARC, which sold naval guns to Singapore. The guns were shipped on to Iran, in breach of UN and British government arms embargoes. Minutes of board meetings at which he was present, and reports to the board, contained details of the £15m contract, known as Project Lisi. He said he had no knowledge of the project or that the guns had gone to Iran.
Labour yesterday said unless Mr Aitken made good his threat to sue World in Action, his crusade against the "cancer of bent and twisted journalism" would be undermined. The Labour frontbencher Donald Dewar said: "The allegations against Mr Aitken ... strike directly at his suitability for high office. If Mr Aitken wishes his stand to carry conviction, a writ against World in Action seems inevitable."
Sources close to Mr Aitken said there had been many calls of support to his office from MPs, Conservative activists and members of the public. Some Tory supporters were clearly delighted, believing a counter-attack on the media was long overdue after months of press attacks endured by ministers. Conservatives felt "enough is enough", said John Thomas, chairman of Mr Aitken's Thanet South constituency Conservative association, which gave him a vote of confidence.
His stand led to renewed calls by Tory MPs for action to protect privacy, but Stephen Dorrell, Secretary of State for National Heritage, made clear his determination to resist the clamour for legislation.
Mr Aitken's aides dismissed suggestions by Menzies Campbell, the Liberal Democrat spokesman, that he should stand down to clear his name. He has already started work on the public spending round for the November Budget.
A source said: "He will get on with the job of controlling public expenditure. The law must take its course. The suggestion that the two are incompatible is preposterous. The idea that you have to resign your job is an absolute disgrace. There will be a strain, but ministers are used to taking difficult decisions."
Jeremy Hanley, the Conservative Party chairman, said: "Jonathan Aitken's right was to take out writs. The Prime Minister has given him support and backing in that."Reuse content