Staff at a restaurant that was caught in the middle of Borough Market terror attack two months ago were left “shaken up” after coming into contact with an unidentified white powder delivered to the premises in an envelope.

Three workers at Feng Sushi, on Stoney Street in south London, suffered skin irritation after opening the package, which contained a powder the restaurant owners likened to a solid fuel such as a firelighter, and was delivered by hand.

The envelope was addressed to the “Feng Sushi team”, and the motive remains unknown. The Metropolitan police said the incident was not being treated as a terror-related.

Feng Sushi, which has eight sites across London that are known for their use of sustainable fish, was one of several premises caught up in the London Bridge and Borough Market terror attack on 3 June, in which three Islamist extremists killed eight people.

That night one of the attackers tapped on the window with a bloodied blade in the middle of the rampage but moved on because the doors had just been locked, according to witnesses.

Chris Mitchell, managing director of Genuine Dining Group, which owns Feng Sushi, said: “Following what happened at Borough Market, anything that goes on feels much more real. They were involved in the terror attacks, they are a little shaken up.”

Mitchell said three staff members had suffered mild skin irritation after being exposed to the substance, and police had taken the package away for examination.

He said the incident had been in other respects uneventful and “quite unspectacular” and the company hoped to reopen the restaurant later on Thursday.

A member of staff at Elliot’s cafe, which is next door to Feng Sushi, said they understood that a package with a harmful substance had been delivered.

The employee said there was a police and fire service presence at the restaurant and a media crowd assembling nearby. Elliot’s closed early because of the “commotion” outside but said the police had not ordered them to shut.

Paul Dawson, a local businessman, said he arrived at Feng Sushi just after the police. “There were two uniformed police in there,” he told the Press Association. “The manager was flustered and said: ‘Sorry, we’ve had something quite bad happen, we have to close.’”

Plainclothes officers, ambulance and fire services then arrived at the scene at lunchtime, he said. “A community warden told me that a white powder had been received,” Dawson said. “I asked him where he had heard that and he couldn’t say.

“Police on the scene didn’t confirm anything, and Borough Market security said ‘some kind of corrosive material attack or incident’. That’s it. Life carried on as normal in the market, which is on its second busiest day of the week.”

No arrests have been made. The London ambulance service said it treated one patient at the scene for minor injuries following a “chemical incident”.

Feng Sushi is majority-owned by Luke Johnson, the entrepreneur and former chair of Channel 4 and Pizza Express.