A former director from a security workers union said on Friday that plain-clothes security guards should only use minimum force when it comes to making legal arrests.

The comment came after a woman said on social media she was accused of theft at a Mannings branch in Tsim Sha Tsui.

She wrote in the post that she paid for the two boxes of cough syrup at a self-checkout counter, but was allegedly grabbed by the neck by two plain-clothes guards and dragged back into the store.

The store apologised to the woman in a statement, saying that the guards from a third-party company had been fired.

It had also ended the contract with the security company.

The police have received the report filed by the woman. The case has now been classified as common assault.

Speaking on an RTHK radio programme, Kevlin Yuen, former director of the Hong Kong Buildings Management and Security Workers General Union, pointed out that both plain-clothes and ordinary security guards require the same kind of licence.

He said uniformed security guards aim to prevent crimes, while plain-clothes guards aim to not let people discover their existence, unless there is a case of theft.

Yuen, who's also a security trainer, emphasised that plain-clothes guards can only perform their duty of stopping suspects after they leave the store.

They then have to identify themselves, before asking for receipts from the suspects.

Only if the suspects try to run away can the guards chase after them.

"In theory, plain-clothes security guards can only use the minimum force necessary to achieve the aim of arresting suspects - which is not to let them run away," he said.

"You don't have to grab the person by the neck or drag them back to the store. You don't have to confine or lock the person up."

Yuen also said once the suspects give up trying to flee the scene, guards have to stop using force.

On the same programme, lawmaker Peter Shiu from the wholesale and retail sector emphasised that guards could be at risk of breaking the law if they use excessive force.

He quoted reports from the Security Bureau, saying that shoplifting cases have increased in the past six months, especially with the availability of self check-out kiosks.

Shiu said once the guards suspect customers of theft, they should report to the police immediately.

"Their role is supposed to stop such a situation. If they have to use force, they should be careful. Because they should call the police at once when they suspect a customer of shoplifting," he said.

"If they have to stop the suspect, they should only use appropriate force. If their use of force exceeds a certain level, they might be at risk of breaking the law."