Not surprisingly, the scenarios - and questions - are stacking up. Just imagine if, in three or four years' time, the nine-dash line is declared invalid under the UN's Convention on the Law of the Sea. Would China really be willing to remove, for example, the ships it is now stationing at the disputed Scarborough Shoal, which sits within the 200-mile exclusive economic zone that the Philippines claims under the convention?

China, of course, is a signatory to the convention, and while any ruling would be legally binding and carry considerable moral weight in the court of international opinion, enforcing it would be another matter.

"There are some really quite ugly scenarios out there and some of them suggest an increased risk of conflict in the long term," one East Asian diplomat said. "If the Philippines can't realistically enforce any successful claim, you have to wonder how this is going to play out. We're in uncharted waters."

Like many, he was treading carefully, given the rising sensitivity about the prospect of a drawn-out legal case involving China and one of its neighbours.

The first indications of China's approach could come in less than a month. Under the convention's raft of complicated provisions, Beijing has 30 days to nominate its representative to sit on a five-judge panel. That panel must decide whether it will accept the case, a hurdle which could be difficult to clear.

Official comments from Foreign Ministry and embassy spokesmen suggest Beijing is in no mood to dignify Manila's action with a detailed response at this point. Their statements reiterated China's "indisputable sovereignty" over the islands of the South China Sea and their adjacent waters and its view that disputes over them must be settled in one-on-one talks.

Chinese scholars say the Philippines should be extremely careful about launching arbitration action without China's consent - and that Beijing should not be bound by the result.

"It is really hard to believe the Philippines is fully prepared to take action in this way and it will only add fuel to the fire," said Professor Zha Daojiong, an international relations scholar at Peking University.

He said if Beijing and the Chinese people were expected to accept a ruling from arbitration they did not accept in the first place, "it could start the unravelling of such a UN process in an Asian context".

Zha urged Manila to return to the "quiet diplomacy" that was the norm across Asia. Manila's "rowdy" use of the international media, and now an attempt at unilateral legal action, indicated a faith in style over substance that could only heighten tensions.

"It is eroding any sense on the Chinese side that they are serious," Zha said.

His Peking University counterpart Professor Chen Shaofeng said no arbitration should proceed unless both parties approved it. He said it was unlikely Beijing would agree to such a process or accept the results of a tribunal it did not recognise.

"There is no precedent in Chinese history of China allowing international arbitration on territorial disputes, no matter over land or waters," Chen said. "The Philippines knows its proposal for arbitration will get nowhere in the end, but it just wants to make the issue more internationalised."

It is unusual for one state to take another to arbitration without its consent. The Law of the Sea is unique among UN conventions in allowing such a step if the two sides cannot come to an agreement. Unlike the UN's International Court of Justice - already rejected by Beijing as an option for the South China Sea - a tribunal cannot rule on sovereignty but can be used to settle a range of related disputes.

China signed the Convention on the Law of the Sea in 1994 but - significantly - exempted itself in 2006 from disputed action in certain areas, including military activities, the delimitation of maritime boundaries and historic waters. Scholars note, however, that considerable legal leeway remains.

Unilateral action against China would still be possible concerning fishing rights in another country's economic zone or if a disputed rock or reef could be considered an island, and therefore entitled to an economic zone - claims that could effectively crack open the nine-dash line, as the Philippines hopes.

China still has several options. One Washington lawyer with experience of the convention said while China could be expected to ignore the Philippines' efforts by refusing to appoint an arbitrator, it might choose to stand and fight on the basis that the exemptions made in 2006 are enough to halt the case. Those exemptions would also give an arbitration tribunal pause for thought about whether it was worth proceeding.

The lawyer said he would be surprised if the tribunal took up the case.

"While the Philippines' statement of claim insists they require no decision on delimitation, a tribunal that was wary of wading into a politically sensitive minefield would likely have little difficulty reaching the contrary conclusion, and finding that China's [exemptions] precluded jurisdiction," he said. "All this would take a year or two to unfold."

The Philippines' statement of claim - crafted with the help of a top-drawer Washington law firm - certainly appears broad, going far beyond the most recent dispute over Scarborough, known in Chinese as Huangyan.

The 20-page claim handed to ambassador Ma and the UN not only calls for the nine-dash line to be declared invalid but says China has unlawfully occupied several features that are part of the Philippine continental shelf and unlawfully exploited the resources of its economic zone.

It describes the failure of 18 years of bilateral diplomatic efforts to find a solution, and concludes that Chinese violations of Philippine sovereignty have only increased in the past year. It points not just to Scarborough but the creation of Sansha , a city under Hainan province, to administer the South China Sea territory China claims.

If Manila is seeking strength through specifics, lawyers and legal scholars note that China could instead be relying on strategic ambiguity for protection.

The nine-dash line - a late 1940s creation by Kuomintang admirals - predates the convention, and Beijing has never precisely spelt out what it signifies or how it fits into the convention framework.

As the Washington lawyer said: "Think of it as a classic use of ambiguity in a strategic and legal sense - it is very hard to prepare to fight in court against something you don't fully understand, and China will one day be able to use that to its advantage."

While regional diplomats are wary about commenting, the region is abuzz with talk of how it will play out.

Professor Carl Thayer, a scholar at the Australian Defence Force Academy, said that if the "bold move" was successful, it could embolden Vietnam and Malaysia to launch their own actions.

"The Philippines has opened a legal front against China's wishes ... China's demand that the South China Sea not be internationalised is now dead in the water," he said.

As it sharpens its legal brief, Manila is bracing for economic and diplomat fall-out from Beijing.