He told a High Court libel jury that "to be thought to be acting maliciously against the Conservative Party would be barmy". it would put off a lot of Guardian readers and cut off the paper's relationships with the government of the day and the Conservative Party.
He denied that the newspaper was engaged in a "malicious crusade" against the Tories.
Mr Preston, shortly to become the editor-in-chief of both the Guardian and the Observer, was giving evidence on the third day of a libel action brought by the Tory party's director-general Paul Judge, 45.
He is suing Mr Preston, the journalists Paul Brown and David Pallister, and Guardian Newspapers, for damages for the harm he says has been done to his reputation by the September 1993 article headlined "Tories face court over Nadir cash - Central Office `tricks' obstructing inquiry".
The hearing was adjourned until today.Reuse content