Voigt had been scheduled to play the lead in a summer production of Richard Strauss' "Ariadne on Naxos," but casting director Peter Katona decided that a slimmer singer would be better for the part, spokesman Christopher Millard told The Associated Press in a telephone interview.
Katona had selected a black evening dress for the part and believed Voigt would not look right in it, Millard said.
"Normally Ariadne is presented on a stylized Greek island with the singers wearing toga-type clothes, but we wanted to present it in an elegant, modern evening dress," Katona was quoted as telling The Sunday Telegraph newspaper.
Anne Schwanewilms, a more slender but lesser-known soprano, is now to sing the part of Ariadne.
Voigt's weight remains a closely guarded secret, the newspaper said, but observers estimate it to be more than 200 pounds.
"I have big hips, and Covent Garden has a problem with them," she was quoted as saying. "Or at least their casting director, Peter Katona, has the problem, and he's made it clear that I won't be singing in his house as long as he's around. Which is sad."
Katona said: "In making these kinds of decision, it is not just a question of how someone looks; it is also how they move on stage," the newspaper reported.
Millard confirmed by telephone that size was the reason that the 43-year-old Voigt, one of the world's most sought-after sopranos, was no longer scheduled to sing in the Royal Opera House production.
He said no contract was canceled in the decision to drop Voigt from the theater's schedule, but would not provide details of any arrangement.
But Voigt, who could not immediately be reached for comment, was quoted as telling The Sunday Telegraph she did have a contract with the theater.
"It's been a legal issue between us, because I was booked for Ariadne, but then he canceled my contract on the grounds that I wouldn't fit the context of the production," Voigt was quoted as saying.
In an interview last month with the Andante recording company's magazine, Voigt reportedly expressed outrage at Covent Garden's decision.
"You know I believe this attitude towards heavy people is the last bastion of open discrimination in our society," Andante quoted her as saying. "Get this: The management of Convent Garden just released me from my contract for Ariadne auf Naxos in 2004. They simply said I was too fat! It makes me so angry."
Several phone calls to her agent in New York City went unanswered, and a message was left on his cell phone.
Voigt regularly appears at the Metropolitan Opera House in New York City. She is to perform there with Placido Domingo in a production of Richard Wagner's "Die Walkuere" this spring. Later, she will sing at the Vienna State Opera.
Three years ago, Voigt sang at Covent Garden, playing the empress in Richard Strauss' "Die Frau ohne Schatten."