Pret a Manger is buying food chain Eat in what its chief executive says is an effort to "turbo-charge" its Veggie Pret chain.
Clive Schlee said the purpose of the deal was to meet demand from vegetarian and vegan customers.
Pret, which has 400 stores in the UK, has four branded as Veggie Pret and aims to convert as many of the 90 Eat stores as possible to the new brand.
The two companies are private equity-owned and no price was disclosed.
The first Veggie Pret was opened in September 2016 as a month-long pop-up in London's Soho, but it ended up becoming permanent.
Mr Schlee later wrote in a blog that the Veggie Pret had been opened as an "experiment, never imagining it would be around for more than a month".
The first one outside London was opened in Manchester last year. Its expansion comes at a time when plant-based foods are increasing in popularity.
"The acquisition of the Eat estate is a wonderful opportunity to turbo-charge the development of Veggie Pret and put significant resources behind it," he said.
Shares in US company Beyond Meat soared at this month's stock market debut.
Pret said it would notify the Competition and Markets Authority of the deal.
Pret a Manger was sold by Bridgepoint to Luxembourg-based JAB Holdings last year. As a result, all 12,000 staff employees received a £1,000 bonus.
Loss-making Eat was owned by Horizon Capital and it is not yet clear what the implications are for its staff.
Last year, Pret said it would list all ingredients, including allergens, on its freshly made products following the death of a teenager who had an allergic reaction after eating a Pret sandwich.
Natasha Ednan-Laperouse, 15, went into cardiac arrest on a flight after buying a sandwich at Heathrow Airport in 2016.