Use your fingerprint to buy a bowl of noodles for half off at 10 yuan ($1.60).

It was a proposition attractive enough to send Su Weiting out of the Shanghai noodle shop where she was getting lunch and around the corner to sign up for an account with yet another payment service provider, Live By Touch.

When I used to buy something that costs over a thousand yuan, I would have to take a wallet bursting of cash, said Su as she finally settled down with her noodles. Using a bank card made things much easier. Now I don't even need that. I can leave home without anything and bring things home.

As consumption booms, especially through the Internet, China's electronic payment service industry has taken off. With credit and debit cards, prepaid cards, mobile payments, online payments, and now biometric payments, dozens of companies are vying for a part of the fast growing pie.

It's a cut-throat industry where deep discounts, presents, and points are luring new clients. Live By Touch, a small player so far operating only in Shanghai, links clients' fingerprints with their bank accounts -- call it the new Midas touch.

Last year, transactions through online payment companies, the PayPals of China, passed 1 trillion yuan ($156 billion) for the first time. And by 2013, it's expected to triple, according to research firm Analysys International.

Cards, which make up the bulk of e-payments, swiped 10.4 trillion yuan last year in retail sales, making up about 35 percent of retail spending, according to the central bank. That in a country a decade ago was mostly a cashed based society.

The numbers can make overseas investors salivate, but it looks, once again, like they could be left in the cold.

While the e-payment service industry outside of debit and credit cards had largely been unregulated, the central bank last year implemented a rule that requires third party payment service providers to get a license.

And it said companies backed by foreign money needed special approval from the State Council, which has been interpreted by some local media as a move to kick out foreign investors.

SENSITIVE TOPIC

It's difficult to say what the government's policy is going to be in terms of enforcement, because they haven't really had any chances to enforce things yet, said Dave Carini, managing director of China Maverick Research.

It's been a sensitive topic and several companies with foreign shareholders have declined to talk.

The rule has also caused a public spat between China's e-commerce giant Alibaba Group and its shareholders SoftBank Corp and Yahoo! Inc.

Alibaba cited the rule as a reason for transferring China's biggest online payment company Alipay, which has nearly half of the market, to a company wholly owned by its chairman Jack Ma. Yahoo and Softbank together have a fully diluted stake of about 69 percent of Alibaba Group.

In the end, the companies agreed that Alibaba Group would receive between $2 billion and $6 billion if Alipay is sold or listed, which some analysts said short-changed Yahoo! and Softbank.

No one would have anticipated that 15 years ago that China would be the world's largest internet market, one of the world's largest e-commerce markets and have 750 million, 800 million people with cell phones, said Gary Rieschel, managing director of Qiming Venture Partners. So a lot of the rules are reactive as opposed to prescriptive.

On some level, it shouldn't be a surprise that the door is now closing on foreign investors in the e-payment industry.

China, like other countries, has long sought to preserve domestic dominance in key sectors, and the financial industry has been one of them. In the credit card segment, the likes of Visa Inc and MasterCard have long been barred from entering the domestic yuan card market despite China's pledge to the WTO to open the payment service market by the end of 2006.

Earlier this year, the U.S. Trade Representative's office requested the World Trade Organization establish a dispute settlement panel in its e-payment case against China to pry open China UnionPay's monopoly on the domestic card market.

I'm very hopeful China will eventually open up, said Ling Hai, MasterCard's Greater China division president. I can't comment on the timing but I think that's definitely the direction it is going.

Former U.S. Congressman and former WTO judge James Bacchus, said he believes the United States will win the case, and that China will have to implement the rulings or face possible economic sanctions.

I think (the plaintiff countries) would be likely to act on this threat of economic sanctions if China did not comply because of the stakes in this particular case, said Bacchus.

It is certainly not in the interest of the United States or China's other trading partners to allow China to allow China UnionPay to develop its own monopoly perch here in China and then use that to secure market access overseas.

The WTO case against China comes as UnionPay steps up its efforts to chip away at Visa and MasterCard's global market.

The Chinese card mammoth is already in over 110 countries and claims to be the third most used card in terms of transaction amount. And footloose Chinese travelers, most of them carrying a UnionPay card, are speeding things up.

UnionPay, majority owned by China's Big Four lenders, had more than 1.4 billion debit cards in circulation last year and was valued at around $11.3 billion in July.

Last year their transaction volume overseas increased about 47 percent, so they're making a lot of headway internationally, said Ben Cavender, associate principal of retail consulting firm China Market Research Group.

If you look at Harrods in the UK, they installed UnionPay pay points. First quarter of this year they've already seen sales to Chinese consumers go up 40 percent.

The UnionPay logo is also finding its way into the wallets of some unsuspecting non-Chinese. One Irish national living in Hong Kong found her bank card, which allowed her to access cash back in ATMs in Ireland before, stopped working last summer. She flipped the newly issued card and found the Plus logo replaced with the tricolor logo of UnionPay which can't yet be used in Ireland.

($1 = 6.399 Chinese Yuan)

(Additional reporting by Lucy Hornby in BEIJING; Editing by Lincoln Feast)