Mr Aitken said he "very much regretted" that he had not disclosed the investment, by a company owned by Saudi Prince Mohammed, to fellow TV- am directors, its chairman Peter Jay, as well as the Independent Broadcasting Authority.
Mr Aitken said he had "made mistakes" in some commercial transactions during his parliamentary career.
But he stated he was a product of the political culture of the time when outside business interests were "considered normal, and if anything encouraged".
The one-time defence procurement minister and chief secretary to the Treasury, said that "in the climate of the Nolan and Downey investigations into MPs' conduct, he would have handled certain things differently.
He told the court that outside interests "were considered quite normal and, if anything, encouraged during my first 10 years as Member of Parliament. But he went on: "We now have a situation in which Nolan came about in 1992, and Sir Gordon Downey has recently said MPs should not have any outside interests at all.
"The climate changes. I would certainly on something like the TV-am stake, have handled it quite differently if I was coming into it in today's climate."
Mr Aitken is suing the Guardian newspaper and Granada Television, makers of the World in Action programme, over allegations that he was in the "pocket" of powerful Saudi interests, pimped for Arabs, and took part in secret arms trading.
George Carman, QC, for the Guardian and Granada, accused Mr Aitken of entering into a secret deal with Prince Mohammed's company to give it control of TV-am.
In 1981, Mr Aitken, an executive with Aitken Telecommunications Holdings Ltd (ATHL), signed an agreement under which Al Bidad, a company owned by Prince Mohammed and in which Mr Aitken was the managing director, invested pounds 2.1m into ATHL.
The money was used to buy into the fledgling TV-am, and over the years the Saudi company injected a total of pounds 3.3m into the television company, the court was told.
Mr Aitken yesterday denied to Mr Justice Popplewell, who is hearing the libel action without a jury, that the Saudis were given control of TV- am in return.
He claims the agreement giving them voting rights was "null and void, and never implemented."
Asked by Mr Carman how a co-director of Al Bilad - a Dr Somait - had believed that the agreement had given the Arab investors majority voting rights, and had indeed come into force, Mr Aitken claimed that the man was "confused both in law and language as to what had happened."
However, Mr Aitken did admit keeping the Saudi investment concealed, and he told the court: "I agree I fell short of good candid business practice, and I apologise for that."
The hearing was adjourned until today.Reuse content