The police are investigating threatening letters sent to four Cabinet ministers under the name of the chairman of the Sungei Road flea market's association, The Sunday Times understands.

They include Prime Minister Lee Hsien Loong and Deputy Prime Minister Tharman Shanmugaratnam.

"The police confirm that reports have been lodged.

"We are unable to comment further as investigations are ongoing," the Singapore Police Force said in response to queries.

It is understood that the letters were all written in English and had hell notes attached to them.

Police visited the one-room MacPherson flat of Mr Koh Eng Khoon, chairman of the Association for the Recycling of Second Hand Goods at Sungei Road flea market, at around midnight on Thursday.

Mr Koh, 76, whom friends refer to as "Ah Koon", told The Sunday Times in Mandarin that two plainclothes police officers went to his home, where he lives with his wife, who works as a cleaner.

He denied any involvement.

Mr Koh said: "I was shocked. I couldn't understand who would want to sabotage me and use my name.

"The police took photos, searched my things and opened my cupboards. They asked if I knew how to write in English but I don't. I didn't even know about the existence of such a letter."

His phone was also taken away by the police, he said.

Mr Koh said he went to a police station near his home and made a statement on Friday afternoon.

"I told them to please investigate and to clear my name."

He had started a petition in late February urging the authorities to allocate an alternative or temporary site to the market, which is set to close from July 11 to make way for future residential developments.

About 4,000 signatures have been gathered.

Mr Koh's association, which represents about 70 of 200 vendors at the flea market, was set up in 2012.

The Government has said the market will not be relocated.