Mr Warren said he had instituted the action over comments made on television by Mr Marsh, 34, the former world light-welterweight champion, because he 'wanted the record put straight'.
He told the High Court in London that he was 'sick and tired' of claims that he knew Mr Marsh had epilepsy in advance of newspaper reports in September 1987.
He said he was 'not in it for the money - if I was, I would have sued Thames Television'.
Mr Warren, 40, of Tewin Wood, Hertfordshire, has said he was 'stunned and puzzled' by the epilepsy revelation as Mr Marsh had just signed a contract to fight an American, Frankie Warren, for more than pounds 100,000.
He was angered by the interview at the centre of the libel case, on Thames Television's Midweek Sports Special in January 1989, when Mr Marsh, asked whether Mr Warren had known about his condition, said: 'I told everybody who needed to know.'
The implication that he had allowed Mr Marsh to sign for the Frankie Warren bout knowing he was suffering from epilepsy was a 'very grave and wounding' libel, he said.
Asked by the judge, Mr Justice Drake, why he did not tell Thames he would sue if it broadcast the interview, Mr Warren said: 'I told them my position. I had a very good relationship with them. I said to them basically, 'Please yourself if that goes out, but I will sue Terry Marsh whatever happens'.'
Mr Marsh, of Lee Chapel South, Basildon, Essex, denies libel and says his comments were true.
He claims he told Mr Warren about the epilepsy diagnosis on two occasions before the September 1987 contract was signed and Mr Warren had told him to keep quiet about it.
Mr Warren said he had launched about 30 previous libel actions and had succeeded in all of them by receiving apologies and damages. He had won pounds 35,000 in a settlement with the Sun in relation to the epilepsy reports, and had received pounds 5,000 from the Sunday Express magazine which, in December 1987, quoted Mr Marsh as saying Mr Warren was either 'deaf or lying' if he said he did not know about the diagnosis.
Mr Warren said: 'If someone had told me a boxer, a world champion, had epilepsy, how could I distance that from my mind? If a boxer I managed disclosed that, obviously the main consideration would be to get him to a doctor and notify the British Boxing Board of Control.'
Mr Warren said Mr Marsh's claim that he had asked him to keep quiet about the epilepsy diagnosis was an 'absolute lie'.
Under cross-examination, Mr Warren denied that he got Mr Marsh to sign the September 1987 contract because he knew of the epilepsy diagnosis and wanted to protect himself in case it became public and put a 'spanner in the works' of the Frankie Warren fight.
During yesterday's exchanges, the jury was told that Mr Marsh had stood trial at the Old Bailey in November 1990 for the attempted murder of Mr Warren, and was acquitted.
The hearing continues today.