Quoting sources, it said: ''All the other people involved in the case had previously been caught by state security departments.'' Again there were no details on how many and who they were.

Xi was among three people invited to Beijing to interview patriarch Deng Xiaoping's handicapped son, Deng Pufang, who is president of the Chinese Federation for the Disabled.

The interview, conducted last week, was published yesterday.

Ming Pao Executive Chief Editor Paul Cheung Kin-bor said last night he did not know why Xi had been arrested.

''We're now trying to find out what happened and we won't comment on the incident before we know more about it,'' he said.

Xinhua said the case was still under investigation.

Xi, in his 30s, was born on the mainland. He is believed to have worked for mainland news organisations before he came to Hong Kong about two years ago.

Colleagues said he made frequent trips to China and had good contacts with officials.

It is not known exactly which articles carried by Ming Pao have been seen by officials as containing ''banking secrets''.

The newspaper, however, has published a series of reports on the shake-up of the People's Bank of China in July and August, including the sacking of former governor Li Guixian and the naming of Vice-Premier Zhu Rongji as the new bank chief.

British Ambassador to Beijing, Sir Robin McLaren, said inquiries about the case would be made as soon as possible.

An official from the Public Security Ministry repeated the Xinhua report but refused to comment further.

A vice-director of Xinhua's Hong Kong office, Zhang Junsheng, said the arrest of the Ming Pao reporter would not affect press freedom in Hong Kong.

''Have any of you who have covered activities in Beijing frequently been caught? How would any of you be arrested if you haven't violated the law?'' he asked reporters.

Mr Zhang said that since Xi worked in Hong Kong, he would be treated as a Hong Kong reporter.

Asked how a reporter could tell whether a piece of information was a state secret, Mr Zhang stressed reporters should be able to make a judgment based on their experience.

A vice-chairman of the Hong Kong Journalists' Association, Kevin Lau Chun-to, expressed concern about the detention, and said the organisation would do its best to find out what had happened and to help Xi.

Legislator Martin Lee Chu-ming urged the Chinese authorities to publicise details of the detention. He also called on Beijing to be lenient and allow Xi to return to Hong Kong as soon as possible.

A mainland analyst said the detention was a warning to the dozens of journalists who had worked for the mainland media before joining Hong Kong newspapers in the past few years.

''These journalists have had good contacts with mainland cadres and can break news Beijing might want to keep under wraps,'' he said.

A vice-chairman of the pro-Beijing Democratic Alliance for the Betterment of Hong Kong, Tam Yiu-chung, urged Hong Kong journalists to be careful and to abide by the law when working on the mainland.

''I hope the Chinese Government will be lenient when dealing with the journalist,'' he said.

A People's Daily editor was sentenced to life in jail last month for allegedly selling a copy of a speech - given by Communist Party chief Jiang Zemin at a party plenum last year - to a reporter, Leung Wai-man, from the Chinese-language Express Daily.

Asked whether Ming Pao had any policies about paying for news, Mr Cheung said: ''We don't agree with that kind of journalism.''