Pret a Manger is reportedly planning to buy rival Eat and turn the chain's 94 stores into vegetarian outlets.
Pret, which has 400 stores in the UK, has four branded as "Veggie Pret" and is aiming to jump on the trend for vegetarian and vegan foods.
It comes as plant-based foods are increasing in popularity, with shares in US company Beyond Meat soaring at this month's stock market debut.
Both Pret a Manger and Eat declined to comment.
The Evening Standard reported that talks between the two chains were understood to be at an advanced stage, with Pret in line to buy the majority of Eat's 94 shops or the whole business.
The paper reported that in February Eat's private-equity owner Horizon Capital appointed Spayne Lindsay, a corporate advisory firm, to offload the business.
Both food chains are owned by private equity companies.
Pret a Manger was sold by Bridgepoint to Luxembourg-based JAB Holdings last year and as result all 12,000 staff employed at Pret received a £1,000 bonus. It has 500 stores globally.
The Financial Times reported that "nothing had been signed" but that the "details were being ironed out" on a deal. It also added that there were potential other bidders for Eat.
In April, Eat reported a pre-tax loss of £17.2m in the year to end of June 2018, but said that it had since experienced 11 consecutive months of like-for-like sales growth.
Veganism is becoming more popular in Great Britain.
Research by the Vegan Society in 2016 estimated there were about 540,000 vegans in the country, compared with about 150,000 in 2006.
Shares of vegan burger maker Beyond Meat soared on their Wall Street debut earlier this month as investors bet on the growing popularity of plant-based foods.