In a move worthy of the rivalry between the Montagues and the Capulets, LVMH Moët Hennessy Louis Vuitton, the world’s largest luxury group, said on Monday that it had entered into a new marriage with Stella McCartney, the British fashion house that was half- owned by its chief competitor, Kering, until last year.

The deal calls for Ms. McCartney, the house’s founder, to continue in her role as the brand’s creative director, and as the majority owner of the Stella McCartney business. She will also become a special adviser to Bernard Arnault, LVMH’s chairman, and to the LVMH board on the topic of sustainability, or how the fashion industry can reduce its famously negative impact on the environment.

Financial details of the partnership were not disclosed. The full scope of the agreement is to be disclosed in September.

Mr. Arnault said in a statement that Ms. McCartney’s longtime commitment to sustainability and ethical issues had been “a decisive factor” in bringing the parties together. He also cited the company’s creation of its own sustainability department more than 25 years ago. “Stella will help us further increase awareness on these important topics,” he said .