Disaster again struck the Miss World pageant yesterday when 18-year-old Miss Germany, Gabriella Brum, renounced the 1980 Miss World crown just 17 hours after winning it in an internationally televised competition at the Royal Albert Hall here.

Contest organizer Julia Morley refused to disclose why Brum suddenly quit after an hour-long conversation with her at the contestants' hotel yesterday afternoon. But it apparently had something to do with Brum's boyfriend, 52-year-old Hollywood film cameraman Benno Bellenbaum, with whom Brum has been living in Los Angeles for the past six months.

Both Brum and Bellenbaum told reporters he was unhappy she had won the beauty contest because she would be separated from him by pageant officials during a year of promotional chores, which can earn an energetic Miss World more than $100,000. Contest officials also were embarrassed by Bellenbaum's revelation to reporters that Brum had posed for nude photographs by him.

Brum left London immediately after her abdication without talking to the press. Morley refused to answer reporters' questions about Bellenbaum, nude photography or what she discussed with Brum, saying "I respect other people's confidences."

Morley said a new Miss World 1980 will be named Monday: It is expected to be the runner-up in Thursday's competition, 18-year-old Miss Guam, Kimberely Santos. She already was aboard a plane on a 10-hour flight to San Francisco when Brum made her unexpected decision to give up the crown.

Brum is the third Miss World in eight years to fail to serve out her reign. Miss United Kingdom, Helen Morgan, quit four days after becoming Miss World in 1974 when it was discovered she was an unmarried mother. American Marjorie Wallace was forced to give up her Miss World crown after 104 days in 1973 for "failing to fulfill the basic requirements of the job" in the conduct of her private life.

Morley said yesterday's melodrama was "not exactly something to cheer with relief about. A lot of the other girls are feeling depressed and sad about it."

Several other contestants said Brum was no Miss Congeniality to her competitors. "She was arrogant and rude and did not mix with the other girls," said Miss Ireland, 21-year-old Michelle Rocca. "But Miss Guam, whom I hope will succeed Miss Germany is a very nice girl and would do the job well."

Miss U.S.A., Brooke Alexander, 17, described Brum as "selfish," rather aggressive" and difficult for her roommate, Miss Austria, to live with."The girls were not happy with her selection," Alexander said.

The six-foot-tall Brum was a teenage model and frequent beauty-contest entrant in Berlin, where she met Bellenbaum last year on a blind date arranged by a mutual friend. He told reporters that "it was kind of love at first sight" and Brum moved to California to live with him after being expelled from school in Germany for reasons neither he nor she would discuss.

"The worst thing I ever did was letting her go in for this competition," he told a reporter for London's Evening Standard. "It seems like she is going into a kind of jail."

Brum began the first -- and last -- day of her reign as Miss World with smiles and modest comments for reporters at a publicity-stunt breakfast with the lord mayor of London at his official residence. She said Bellenbaum was "not too pleased" when she telephoned him with news of her victory. But to other questions about their relationship, she answered, "What I do in my private life is nothing to do with anyone."

She then met with Morley, who announced that Brum "has told us that she does not wish to hold the title of Miss World 1980 and, having discussed the matter with her, we have agreed to respect her request. She will now get none of the prize money."