MILAN, Italy — Yoox Net-a-Porter SpA climbed on its debut following the merger of Yoox and Net-a-Porter, which formed the world's largest online luxury retailer.

The shares gained 2 percent to 28.63 euros as of 9:02 a.m. in Milan, giving the company a market value of about 3.7 billion euros ($4.2 billion).

Yoox agreed in March to buy Net-a-Porter from Cie. Financiere Richemont SA in an all-stock transaction. The combined entity has annual revenue of 1.3 billion euros ($1.5 billion) and more heft in the market as competition intensifies in Web retailing of luxury goods. Yoox founder Federico Marchetti is the new company's chief executive officer, while Net-a-Porter founder Natalie Massenet, who was set to be executive chairman, quit last month.

Richemont, which owns about 50 percent of Yoox Net-a-Porter stock, also gained in early trading. The maker of Cartier watches said earlier full-year profit will be boosted by an accounting gain of 610 million euros to 670 million euros from the transaction, based on Friday’s closing share price.

Shares of the Geneva-based luxury-goods maker rose as much as 4 percent.

Richemont also said it agreed not to sell a 25 percent stake in the new company during three years.

By: Andrew Roberts; editors: Matthew Boyle and Thomas Mulier.