Can an employer withhold the 13th month pay of an employee because she is resigning?

Hi,
I have worked for a bank since March 2010 until present. However, I have tendered a 30-day notice of my resignation which will be effective on December 20, 2011.

The company's way of giving the 13th month pay is every Nov. 29th and Dec. 14th. However, they already withheld the 1st part of my 13th month pay because I am resigning.

Is this legal for them to do? Based on PD 851, an employee can demand the 13th month pay from the employer during the effectivity of his resignation. But when I called DOLE, they told me to wait until I am cleared.

This bank that I am working for is not so fond of resigned employees. It takes them more than six months, and even more than a year to release an employee's final pay. For this reason, I want to know if I have strong grounds to demand for the payment of the 13th month pay. According to PD 851, I can definitely do this but I am not quite sure about how this decree is being implemented in the real world.

13th month pay, last days payment, withholding tax refund and SL/VL (5days each) are mandatory monetary claims that is due to you. Now, wait for one month processing and inquire for it. If they insist that it takes longer, file SENA Money claims at DOLE office. Mabilis ang collections mapahiya pa company mo before. Play with them.