Last year, for the first time in its history, the retailer began selling brands in 19 stores in the Northeast in a bid to draw shoppers into its stores.
The experiment will now be replicated in the South, suppliers told The Financial Times.
Marks & Spencer has identified shops in northwest London and Hertfordshire to carry the 350 brands, which include Tetley tea.
The decision to stock brands was seen as a way to improve the performance of M&S's food busines.
Last July M&S executive chairman Sir Stuart Rose ousted food boss Stuart Esom and installed John Dixon, who has focused on improving availability, innovation and promotions such as "Dine in for£10".
Other recent changes in Marks & Spencer’s senior food team include the appointment of Woolworths’ South Africa joint head of food Cathy Chapman as head of product development and direction, and Asda’s private label director Andy Adcock as food commercial director.