''The way the Chinese authorities treated Xi Yang's case was very frustrating. It has also raised questions about whether some guidelines are needed to protect oneself against unknowingly trespassing on Chinese law,'' said one partner at a major law firm based in Hong Kong.

He said he used to bring the documents back to Hong Kong for further reading. But now he prefers to read them in China or simply ask local lawyers to deal with the documents.

But another partner at an accounting firm which deals with H-share listings said asking mainland lawyers to endorse ''sensitive documents'' could be inefficient and time-consuming.

A stocks analyst covering China said he needed to ask Chinese officials about various macro-economic policies.

''Would the action constitute stealing or spying on state secrets? In the past, the general thinking was it only involved politics, but Xi Yang's case proves otherwise,'' he said.

''Perhaps the only assurance is the fact that China still needs foreign capital for its development. It is highly unlikely it will do anything to hinder that.'' Shenzhen authorities have pledged to crack down on smuggling of state economic secrets to Hong Kong.

A report published last week by the Hong Kong China News Agency said there had been 20 cases of tourists trying to leave China with classified documents in their luggage between January and March this year.

Shenzhen authorities dealt with 49 official document smuggling cases last year, an increase of almost 100 per cent on the year before.

The agency report said those caught were mostly Chinese citizens seeking to sell the documents to foreign businesses and Hong Kong public servants.

No details were given as to the nationality of those non-Chinese citizens caught, nor what happened to those who were apprehended.