Natalie Massenet, who founded the e-tailer in June 2000, has an estimated 18 per cent stake in the business, which will be sold to Geneva-based luxury goods retailer Richemont.
It already owns 33 per cent of the company, and has made an offer for the remaining shares in a deal valuing Net-a-Porter at about 350m.
Massenet, who will remain as executive chairman, is reportedly reinvesting about 15m of the proceeds back into the business.
She remains "completely committed" to the venture and said yesterday development plans would be announced later this year. Richemont is likely to help the firm pursue growth opportunities in south-east Asia.
"Richemont has completely embraced our vision and strategy since it came on board as a shareholder and together we are going to continue to build the 21st century model for luxury fashion retailing," said Massenet.
Presented in the style of a fashion magazine, Net-a-Porter features collections from some of the world's most coveted designers, including Chloe, Marc Jacobs, Burberry and Stella McCartney.
Net-a-Porter generated sales of 120m in the year to January, and employs almost 600 people in London and New York.
The move comes as speculation heats up that luxury goods companies with strong balance sheets, such as Swatch Group and Hermes, are likely to pounce on targets as the sector recovers.
Jon Cox, an analyst at Kepler Capital Markets, said: "If you believe that the internet for luxury goods is going to be anywhere like the internet already is for high street retail, it looks like a good bit of business."
Today, Net-a-Porter is viewed by more than two million people a month and features collections from over 300 of the world's most coveted designers.