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Natalie Massenet, a former fashion journalist, started Net-a-Porter in 2000 from her London apartment after spying a gap in the market for selling luxury goods online.Credit Jonathan Player for The New York Times

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LONDON — Natalie Massenet, the founder of Net-a-Porter and the woman who persuaded high fashion that it had a home online, is leaving the British luxury e-commerce group she built, just five months after announcing a merger with its Italian archrival, Yoox.

She resigned shortly after selling her shares in Net-a-Porter, which were valued at 100 million pounds, or $153 million.

Ms. Massenet, a former fashion journalist who started Net-a-Porter in 2000 from her London apartment after spying a gap in the market for selling luxury goods online to the world’s wealthiest women, handed in her resignation on Wednesday, Yoox said in a statement.

“Following Natalie Massenet’s tendering of her resignation today as executive chairman of the Net-a-Porter Group, Ms. Massenet will not be a member of the board of directors of the new group which will result from the upcoming merger,” according to the statement.

News of Ms. Massenet’s abrupt departure was first reported by Bloomberg. Hours earlier, Britain’s competition regulator, the Competition and Markets Authority, cleared the way for Yoox to buy Net-a-Porter from its controlling shareholder, Richemont, in an all-share deal worth £936 million.

The combined Yoox Net-a-Porter Group, which is set to list on the Milan stock exchange in October, will become the world’s largest online purveyor of luxury goods and have a projected valuation of 3.4 billion euros, combined sales of €1.3 billion and 24 million users worldwide.

But Ms. Massenet’s departure may raise questions about the future of Net-a-Porter at a time when competition between online players in high-end products has never been higher.

Along with a flurry of new entrants into the realm of luxury fashion, many established offline brands are also taking greater control of their e-commerce operations, putting pressure on new-guard websites in the face of a slowing Chinese economy, geopolitical volatility and currency fluctuations that many fear could curb consumer appetites.

On Tuesday, LVMH Moët Hennessy Louis Vuitton, the world’s largest luxury group, announced it had appointed Ian Rogers, Apple’s former head of music, as chief digital officer, a clear signal that it was serious about its online strategy.

Luca Solca, an analyst at Exane BNP Paribas, said he believed that Ms. Massenet’s departure at this early stage in the merger “would put the company on the back foot.”

“Natalie was probably at the center of Net-a-Porter’s relationships with the luxury brands they sell,” he said.

The designer Diane von Furstenberg, whose brand is sold on Net-a-Porter, expressed sorrow and surprise at the news. “She is the soul of the brand, and whenever a brand loses its soul, it cannot be the same,” she said.

Ms. Massenet’s departure was not entirely unexpected, however. When the deal was made public, Ms. Massenet was given the title of executive chairwoman of the new group, overseeing editorial content and advertising strategy, while Federico Marchetti, founder and chief executive of Yoox, was made the new group’s chief executive.

But in recent months, speculation emerged that Ms. Massenet was unhappy at the valuation given Net-a-Porter by Richemont before the deal. There was also word of disagreements between Ms. Massenet and Mr. Marchetti, who made it clear in subsequent interviews and statements to the media that he was taking control of the new group.

“I do not think they ever got along very well,” said Mr. Solca, who noted that Ms. Massenet was not on the conference call announcing the merger. “It always seemed like a matter of time.”

Neither Ms. Massenet nor representatives of Net-a-Porter returned calls seeking comment on Wednesday.