Jane Ennis, editor of Now magazine, has handed in her notice after almost 10 years at the helm.
Ennis told Press Gazette: "I've had a fantastic run. This has been the best job I've ever had, and I am sad to be leaving a great team."
She told the magazine's staff on Tuesday that she had resigned. She has a six-month notice period, but is expected to leave in February.
Her plans for the future are not known, but a friend said she is looking for a new challenge. Ennis is widely credited with inventing the market for gossip magazines.
During her reign, she has seen the launch of five titles based on her pioneering formula of celebrity gossip and paparazzi pictures. These include Heat, Closer, Reveal, New and Star.
Despite the competition, Now maintained market leadership through eight consecutive ABC periods, finally losing its number one position to Heat last year.
An insider at IPC said: "Jane has been unhappy for some time at the lack of investment in Now. "She has described the experiences of editing the title in recent years as being the equivalent of running up the down escalator."
Her deputy, Jeremy Mark, is among those being considered as a successor for the editor's chair.
Ennis started her career on regional newspapers, before moving to magazines. She worked as a writer on the groundbreaking Nova Magazine and at editor level on Best, TV Quick and The Telegraph Magazine. She launched Here Magazine for G&J, which was later bought by IPC.
She has also had a successful newspaper career, working as features editor on the Sunday Mirror, and assistant editor, features, on the defunct Today newspaper.
Ennis is the third highly rated weekly magazine editor to have resigned from IPC in the past six months. Elsa McAlonan, editor of Woman's Own, and Phil Hilton, editor of Nuts, are also serving notice periods.
IPC said Ennis would be leading Now's 10th anniversary celebrations, as well as overseeing a new look for the magazine, before leaving the title in the New Year.