Pret a Manger's bosses found themselves engulfed by a wave of horror from Britain's chattering classes after the announcement that the American burger giant McDonald's had taken a 33% stake in the upmarket sandwich chain.

Pret said it had sold the stake to help it expand overseas. Andrew Rolfe, Pret's chief executive, said: "The McDonald's investment will provide so much more than just money." Pret could now take advantage of McDonald's expertise and infrastructure around the world, "including their capability in real estate, construction, distribution and business systems," Rolfe said.

However, Pret staff found themselves teased without mercy by customers asking for "a Big Mac and fries, thanks," and press commentators were unimpressed. Jonathan Meades wrote in the Times: "McDonald's plus Pret a Manger equals a scandal! A tragedy! The old world devoured in the new world's rapacious maw."

Cahal Milmo in the Independent said the deal "raised questions about its impact on the Pret name", and quoted the Oxford-based group Corporate Watch, which said: "It must be inevitable that the management practices and employment policies of McDonald's will be passed down to a smaller company like Pret A Manger. Nobody wins from this deal."

In the Guardian, Andrew Clark wrote "For Pret, a company whose success has always been finely balanced on the line between affordability and luxury, environmental awareness and pre-packaged convenience, the deal could yet prove dangerous: the shadow cast by the golden arches is a notoriously long and murky one."

He described a meeting at Fabric, a nightclub opposite Smithfield Market in London, where the sandwich chain's 400 supervisors had been invited to hear its founders, Sinclair Beecham and Julian Metcalfe, announce the deal, and said: "For many staff, the idea of a link with McDonald's, which has been demonised by the green lobby, was horrifying."

In the London Evening Standard, Jim Armitage wrote: "How could they even think of it? Every office worker in the land will be howling in horror and disbelief today, at the news that McDonald's is to take a one-third share in Pret a Manger, the sandwich bar chain that has become a staple for Britain's office workers.

The very idea of the temple of junk food taking a bite out of the inventors of quality sandwiches is enough to make one choke over one's crayfish and rocket."

The paper's news pages quoted a Pret customer saying: "McDonald's is cheap and cheerful. The same is going to happen to Pret. They are going to kill it.

"If they reduce the cost of sandwiches to attract customers, the high quality of food will inevitably suffer. McDonald's and style do not go together."

"The worry is that Pret stores will now start popping up everywhere and you can imagine them appearing even in India and Moscow - everywhere where there's a McDonald's."

Standard columnist Matthew Norman joined the chatter, asking: "I wonder how clever Pret A Manger is to get into bed with McDonald's," and attacking Big Mac's "appealingly semi-congealed 100 per cent pure beef patties" and its "awful public image" after its "persecution through the courts of those two protesters", the work of "an unusually nasty corporate bully", while Pret A Manger, on the other hand, "is an absolutely brilliant business with as good an image as any modern British chain."

However, the Standard's Zoe Williams brought some balance to the debate, writing an open request to Pret's bosses: "Please introduce Bacon and Egg McMuffins to Pret. I love them, but am too ashamed to be seen coming out of McDonald's in broad daylight, and you can't get them at night."