But a proposed law on separatism, announced on Friday, is not aimed at territory already under the sway of the People's Republic.

The target is Taiwan, claimed by Chinese leaders as an inseparable part of the motherland but where the writ of their ruling Communist party has never run.

Beijing hopes that through legislation it can stop what is sees as efforts by Chen Shui-bian, Taiwan's president, to formalise a political divide that has endured since Mao Zedong's revolutionaries chased their Kuomintang rivals into refuge on the island in 1949.

China has given no details of the law's content but some mainland analysts expect it to set in stone Beijing's longstanding threat to invade Taiwan if the island declares independence.

Giving such threats the force of law, they believe, could deter Mr Chen from pushing through plans for constitutional reform that might weaken democratic Taiwan's historical links with China.

"Setting an anti-secession law will help to address the shortcomings of the central government's use of policy announcements and formal statements of position made by spokespeople," wrote the Wen Wei Po, a pro-Beijing daily based in Hong Kong.

"It will leave 'Taiwan independence' forces with no room for ambiguity to exploit," it said in an editorial reprinted by a number of mainland publications.

But introducing legislation to one of the most sensitive policy issues in Chinese politics carries significant risks.

The plan could further alienate Taiwan's voters - Taipei has already reacted angrily, with ruling party politicians and officials denouncing it as an attempt to establish a legal basis for a future invasion.

"They are using this method to announce to the world they will do anything to swallow up Taiwan and will establish the legal basis to do so," Annette Lu, Taiwanese vice-president, said at the weekend.

Tough legislation could upset the US, final arbiter of Taiwan's security. Washington has opposed moves by Mr Chen seen as pushing a pro-independence agenda but on Friday gave a chilly welcome to Beijing's plans.

Richard Boucher, State Department spokesman, said the US was against any change to the status quo. "We think it's the time to focus on dialogue and not for hardening of positions," he said.

And while legislation will underline the importance Beijing attaches to reunification, it could also limit leaders' own room for manoeuvre and compromise on an issue fraught with complexities.

China says a declaration of Taiwanese independence would be cause for war, for example, but Mr Chen and his Democratic Progressive party have long argued there is no need for such a declaration since Taiwan's "Republic of China" state is already independent and sovereign.

It is unclear how the proposed law could prevent Mr Chen pushing such policies as making state companies use "Taiwan" in their names instead of "China" or rewriting schoolbooks to emphasise Taiwanese history.

Some analysts also question why Beijing is introducing legislation now weeks after Mr Chen and the DPP suffered an unexpected setback in elections they had hoped would end relatively pro-Chinese opposition parties' control of the Taiwanese legislature.

In part the decision may stem from perceptions that recent martial mainland rhetoric helped undermine support for the DPP.

But George Tsai, an expert in cross-Strait relations at Taipei's Institute of International Relations, says it also reflects Chinese leaders' deep concern about the emergence of a separate sense of identity in Taiwan.

Still, Mr Tsai hopes Beijing will include carrots as well as sticks in the new law, perhaps by offering incentives for Taiwan to move toward unification.

The law's bottom line is likely to be punitive, however. During a recent meeting with Taiwanese immigrants to Brazil, Hu Jintao, China's president, last month stressed that people from the rival sides of the Taiwan Strait were "one family" - like it or not.

"We Chinese people have always had a tradition ... that anyone who splits the nation is a criminal condemned by history," Mr Hu said. And in future, it seems, by law.

Source: FT