In its first comments about the case, China said on Tuesday that Ching is being investigated for espionage, a charge that can carry the death sentence.
"Ching has admitted that in recent years he has been following the instructions of overseas intelligence organisations and has undertaken intelligence collecting activities on mainland China," the Foreign Ministry said in a statement.
"He has collected a large amount of spy fees."
The statement did not say what countries or overseas groups he had allegedly spied for.
Ching's wife Mary Lau said on Monday that her husband was detained while trying to obtain a sensitive manuscript of secret interviews with former Chinese leader Zhao Ziyang.
Zhao was deposed for opposing the government's decision to send in troops to violently crush pro-democracy demonstrators in 1989.
The former premier and secretary-general of the Communist Party's inside knowledge of what led to the decision that resulted in hundreds, perhaps thousands, of people being killed by troops in Tiananmen Square would be explosive material.
"Ching has admitted that in recent years he has been following the instructions of overseas intelligence organisations and has undertaken intelligence collecting activities on mainland China. He has collected a large amount of spy fees"
Chinese foreign ministry statement
Observers believe Ching's arrest is part of a wider probe by Beijing into who might have access to the manuscript to stop it from being taken out of the country to be published.
The author of the manuscript, Zong Fengming, 85, who had rare access to Zhao while the leader was under house arrest from 1989 until his death in January, said on Tuesday that he has been under pressure by the government to not publish the document.
"They've visited my home a few times ... They told me not to expand Zhao Ziyang's influence," said Zong, who denied knowing Ching.
Straits Times shock
Zong said the chief editor and another employee at the Hong Kong Xinfeng Publishing Company, which published an earlier book last year in which he briefly quoted Zhao, have also been harassed by Chinese authorities.
"They were followed and questioned ... They are afraid to publish the second book," Zong said.
Foreign ministry spokesman Kong Quan on Tuesday said China had "ample evidence" against Ching but refused to say what that evidence was.
"Until we see incontrovertible evidence, we stand by our belief that he (Cheong) has always acted in the best interests of the Straits Times"
Singapore Press Holdings statement
"Ching Cheung's case is not related at all to Zhao Ziyang," Kong said.
The fact that Ching is a Hong Kong citizen, not a Singaporean citizen, could make it difficult for his family to win his release.
The Straits Times' publishing company expressed shock at the charges against Ching, who has been working for the daily since 1996.
"Until we see incontrovertible evidence, we stand by our belief that he has always acted in the best interests of the Straits Times," Singapore Press Holdings said in a statement.