Starbucks boss Howard Schultz has caused a buzz, but not the kind generated by a dose of caffeine.

Guns are no longer welcome in Starbucks shops, >Schultz said an open letter to Americans posted on the Starbucks website.

“The presence of a weapon in our stores is unsettling and upsetting for many of our customers,” he wrote.

The letter came after 12 people were gunned down at the Navy Yard in Washington, the latest in a series of horrifying US shooting sprees, though Schultz did not mention the incident in his letter.

Schultz said he felt compelled to write because Starbucks coffee shops and employees of the company have recently been thrust unwillingly into the US debate about open carrying of firearms.

Schultz explained that Starbucks’ approach to open carry had been to follow local laws. Most states have no restriction on openly carrying a weapon, according to opencarry.org, a social-media portal for the open-carry movement.

But the debate has become increasingly uncivil and, in some cases, threatening, Schultz said. Open carry advocates, for example, have solicited and confronted customers and employees in Starbucks coffee shops, he said.

Pro-gun activists have used Starbucks coffee shops as a political stage for media events, he said.

These have been called Starbucks Appreciation Days, which have misleadingly portrayed Starbucks as a champion of open carry, Schultz said. Starbucks does not want these events to be held in its stores, he noted.

“For these reasons, today we are respectfully requesting that customers no longer bring firearms into our stores or outdoor seating areas — even in states where open carry is permitted — unless they are authorized law enforcement personnel,” Schultz wrote.

Last month, Starbucks closed a coffee shop in Newtown, Connecticut, early after gun rights supporters held one of their events there, The New York Times reported. The town is where 20 children and six educators were killed in a mass shooting December.

Gun-control advocates have attempted to counter the Starbucks Appreciation Days by staging protests outside Starbucks stores and urging consumers to boycott the company. They have also organised “skip Starbucks Saturdays,” urging people not to buy because of the company’s policy toward open carry.

John Pierce, co-founder of opencarry.org, told dpa he was disappointed by Schultz’s letter. It was especially bitter because grassroots organisations advocating open carry “went out of their way” a few weeks ago to support Starbucks when anti-gun activists promoted their skip Starbucks Saturdays.

“I’ve personally unliked their Facebook page, and what I am not doing any more is I am not spending my money there,” Pierce said.

Pierce conceded that Starbucks was within its rights to change its policy, and gun owners should honour the request.

According to opencarry.org only five states — Illinois, New York, Texas, South Carolina, Florida and the District of Columbia — prohibit people from openly displaying their weapons in public.

California generally only allows open carry in places with a relatively low population.

In the majority of the other states, Pierce said any law-abiding citizen who is not prohibited from owning a gun can carry it openly without any Government permission required. Some local Governments have stricter regulations than their states.

Pierce said opencarry.org advocates carry guns in holsters.

The people who carry guns may end up having the last say.

Schultz wrote that Starbucks would not slap an outright ban on guns because enforcing such a ban "would potentially require our partners to confront armed customers."