Starbucks Korea has suffered a “very significant” fall in sales following a controversial marketing campaign that evoked a 1980 military crackdown on pro-democracy protesters, triggering public outrage.

The retailer has faced mounting criticism over its "Tank Day" campaign, which used tumblers to mark the anniversary of the May 18 Gwangju uprising, when the military government deployed troops and tanks to suppress pro-democracy demonstrations.

In a press conference on Tuesday, Chung Yong Jin, chairperson of the American coffee chain’s local operator Shinsegae Group, issued a public apology, urging people not to direct their anger towards Starbucks Korea employees and frontline staff.

"I take it very seriously that Starbucks Korea's inappropriate marketing hurt and angered many people," Mr Chung said. "I will take all responsibility for the incident."

Hundreds of people are estimated ‌to have died or gone missing when the military dictatorship of Chun Doo Hwan cracked down on the protests in Gwangju. Many details remain unconfirmed, including who gave the order to open fire.

Starbucks Korea's e-commerce team organised the campaign and received final approval from team leaders and executives, a company official said.

The investigation had not been able to conclude whether there was intentional wrongdoing, but the incident exposed serious flaws in Starbucks Korea's risk management framework, the company said.

Shares in Shinsegae fell as much as 2.8 per cent in morning trade before reversing course to rise 1.7 per cent.

A Shinsegae official said that sales had fallen sharply since the marketing controversy and an internal investigation was focused on whether there had been any premeditation or intentional wrongdoing ⁠by management or employees.

"While sales are not our main concern at the moment, we have seen a very significant drop," said the official.

The e-commerce team had been overly focused on sales amid a large volume of weekly promotional events, leading staff to approve the campaign without proper review or legal scrutiny, it said.

Starbucks global headquarters in the US ​was aware of the gravity of the situation and had been receiving updates on the investigation ‌and the company's response, it added.

Shinsegae fired the head ⁠of Starbucks Korea last week after apologising over the campaign. Starbucks Global also apologised and said ⁠an investigation had begun.

Starbucks is the leading food and beverage chain in South Korea in terms of the estimated number of customers in the six months to February, according to data firm Wiseapp.

Shinsegae operates Starbucks Korea ‌through SCK Company, which is 67.5 per cent owned by E-Mart ​and 32.5% by Singapore’s sovereign wealth fund GIC, according to a company filing.