A duty-free company that operates at London's Heathrow Airport has apologized after Chinese customers accused it of discrimination.

Chinese social media users alleged Chinese customers had to spend more than 1,000 pounds ($1,387) to qualify for a discount voucher at World Duty Free while other nationalities had to spend 79 pounds.

In response to the allegations, World Duty Free issued an apology on Twitter.

"We're grateful for comments shared on social media today regarding our VIP voucher scheme," the statement said. "We've investigated this straight away and identified an issue which we have corrected with immediate effect. Please accept our sincere apologies."

The company, which is based in Bern, Switzerland, did not respond to requests for a full explanation.

A Heathrow Airport spokesperson said: "We have been made aware of this offer which we find unacceptable. We are currently working with our commercial partner to understand how this happened and ensure it does not happen again."

The allegations surfaced on Weibo, when a part-time Chinese employee at the store at the airport's Terminal 2 known as @Renjiannaipao, published allegations on the social media platform.

The claim that Chinese travelers were being discriminated against went viral with people posting to complain about mistreatment.

@Julialovekshwrote on Twitter: "Will never shop in the duty free store at Heathrow any more. Chinese customers deserve to be treated with respect!! What a hideous and discriminating event to happen in 2018!! Disgusting."

@QianyunZhu_zqy tweeted: "Shame on you World Duty Free! When celebrating Chinese New Year with promotion info in Twitter, please remember Chinese customers are still waiting for your official explanation and apology for the racial discrimination. What a racist and two-faced company."

Increasing numbers of Chinese tourists are visiting the UK and other destinations and they are known for being big spenders.

The number of Chinese visitors to the UK increased by 33 percent in 2017. They collectively spent 667 million pounds, according to UK tourism authority Visit Britain.