American scientist James Watson, who won a Nobel Prize for his groundbreaking work into the DNA, has been stripped of his last honorary titles after making further controversial comments about race.

Cold Spring Harbor Laboratory, which Watson once headed, explained in a statement that its move to strip the scientist of his titles as Chancellor Emeritus, Oliver R Grace Professor Emeritus and honorary trustee, is in response to comments he made in a recently-aired PBS documentary.

In the "American Masters: Decoding Watson" film, the molecular biologist, 90, said that genes cause a difference in intelligence on average between black and white people in IQ tests.

The Lab said it "unequivocally rejects the unsubstantiated and reckless personal opinions Dr. James D. Watson expressed on the subject of ethnicity and genetics."

"Dr. Watson's statements are reprehensible, unsupported by science, an in no way represent the views of CSHL, its trustees, faculty, staff, or students.

"The Laboratory condemns the misuse of science to justify prejudice," it added.

The Cold Spring Habor Laboratory terminated Watson's status as Chancellor in 2007 following similar controversial comments.

Watson had told a magazine at the time that he was "inherently gloomy about the prospect for Africa" because "all our social policies are based on the fact that their intelligence is the same as ours — where all the testing says not really."

He added that while he hoped everyone was equal, "people who have to deal with black employees find this is not true."

Rufus Watson rejected the notion that his father is a "bigot" in a telephone interview with the Associated Press.

"My dad's statements might make him out to be a bigot and discriminatory...(but) they just represent his rather narrow interpretation of genetic destiny."

"My dad had made the lab his life, and yet now the lab considers him a liability," he added. He also said that his father has been living in a nursing home since an October car crash and that his awareness of his surrounding is "very minimal."

Dr. Watson won the 1962 Nobel Prize in Physiology and Medicine alongside Francis Crick and Maurice Wilkins for their discovery, in 1953, that DNA was a double helix and "its significance for information transfer."