Turnover at the first Pod at London Wall, which opened 18 months ago, has just reached the £1m mark.
A new 100-cover site will open in EC3 in October, and a third site is expected to open before the end of the year.
Tim Hall, who founded Pod with former Pret a Manger partner Kate Skerrit, said they plan eventually to roll out the concept nationally.
"Our strategy is to consolidate in the area we know well, the City, but we think long-term opportunities lie more in a high-street environment," said Hall.
"We plan to try places like Birmingham, Bristol or Edinburgh to see how Pod works in a non-London location."
Pod, which bills itself as an eco-friendy, healthy fast-food restaurant, serves more than 1,000 people a day with dishes like Feta, Watermelon and Rocket Salad; and Summer English Pea and Mint Soup.
Hall added: "There's a gap in the market between places like Pret a Manger, and the likes of Pizza Express, and I think Pod sits in the middle."