George Galloway, MP for Glasgow Hillhead, was giving evidence for Tory member Rupert Allason, who is suing Mr Campbell, political journalist Andy McSmith and Mirror Group Newspapers for malicious falsehood.
Mr Allason claims that Mr Campbell, then the Daily Mirror's political editor, and former colleague Mr McSmith, targeted him by falsely writing that 50 MPs had signed an early day motion attacking the member for Torbay over libel damages he had received from MGN. All three defendants deny the claims, which arise from an article in the Daily Mirror on 20 November 1992.
Mr Galloway told of the personal history between himself, Mr Campbell and former Mirror Group owner Robert Maxwell. "My feelings are ... a man who can serve with such gusto the greatest thief of the 20th century in this country is a person to be kept a wary eye on."
He added: "I always kept a weather eye on what he was up to, which was generally no good."
Mr Allason, a spy writer under the name Nigel West, claims that the article was published maliciously and lost him a contract to write a book for the US-based company Holmes Protection Group. The article alleged that 50 MPs had signed the EDM calling on Mr Allason to show his concern for Maxwell pensioners by contributing some of the estimated pounds 250,000 libel damages that he had won from the group to their cause. He says the EDM did not exist until after the article.
The case continues.