A dirty little secret of one Japanese "office lady" stereotype is that some women, dissatisfied with their clerical salaries, augment their earnings by moonlighting as bar hostesses.

The second job is not necessarily one they want revealed — carrying, as it does, images of ladies of the night in flashy dresses and makeup, entertaining salarymen with lascivious lights in their eyes.

But the upcoming introduction of the 12-digit My Number identification code assigned to all residents in Japan threatens to expose their nocturnal identity. Tax professionals say it may force some to consider abandoning their second job, often the more lucrative.

My Number will "first of all make incomes transparent for the tax authorities," said Manabu Kondo, a tax accountant and author based in Kizugawa, Kyoto Prefecture. "Until now, the government may have been able to identify names of bar operators (for tax purposes), but it has had difficulty identifying individuals working for them."

Tax professionals think many of the moonlighting hostesses evade taxes on their secondary earnings, often because they are unaware they are required to file tax returns for their second job.

Tax offices would not bother to track these individuals because of the difficulty of identifying hostesses who use false names, and employers who pay them in cash and employ other tricks to hide transactions, the professionals point out.

In the case of someone working for three employers, My Number can help authorities track all three income sources, Kondo said.

Overcoming years of resistance, the government this month will start sending out a tsuchi kaado (notification card) bearing the personal identification number to all individuals on the residency registry, officially kicking off a project to unify hitherto separate identities in governmental services, including social security, taxation and national health insurance.

The initial phase of the project, from January, will start seeing the numbers used in tax administration as well as social security and disaster relief benefits. This is expected to greatly enhance the efficiency of administration.

With the My Number launch three months away, hostesses should prepare to start filing tax returns because tax inspectors can easily go after them.

But there is a danger the individual should be aware of: After receiving tax returns describing their true incomes, the tax office may relay that information to the municipal government, which would then send a document detailing the total local levies to the daytime employer — because that employer is typically responsible for deducting the amount from wages. The employer might spot discrepancies between the tax sum and the presumably low salary a female clerk, say, might be earning.

Hostesses fear this scenario may lead to their daytime employers, and potentially their colleagues, finding out about their second identity.

And a similar fear is shared by salaried workers who have a second job they do not want their main employer to know about, in a country where devotion to the firm is the norm.

National Tax Agency statistics show that the average annual income of salaried workers peaked at ¥4.67 million in 1997 and shrank thereafter to ¥4.14 million in 2013, likely prompting many workers to look for additional income sources.

But employers remain reluctant to see their staff working elsewhere, with only 3.8 percent of 4,513 companies nationwide saying they explicitly allow their staff to take up a second job, according to a survey conducted by Recruit Career Co. from last November to February this year.

Zenya Ozawa, an author and certified public accountant based in Tokyo's Ginza district, said one way to avoid a secret second job from becoming known is not to give one's assigned My Number to the second employer — albeit not a recommended strategy.

"Do it at your own risk. At least for now, there's no penalty for not providing your My Number to your employer," Ozawa said. "I think that's what hostesses probably will do."

Ozawa pointed to the NTA website, where a "frequently asked questions" section explicitly says there is no penalty prescribed in tax laws for failure to include My Number on submissions to the tax office because the government assumes there will still be people who may not have received their assigned numbers. But it also notes that to enter the number is an "obligation dictated by law" and urges taxpayers to "enter (it) correctly and submit."

Tax accountant Kondo said dual-earners can choose to pay taxes under the self-employed category instead of becoming "employed" by the second employer, which ensures that the local tax amount will not be sent to the daytime employer.