Last day of service

I intend to tender my resignation with effect 31 Dec 2012. However will only work until 14Dec and use offset my unused Annual Leave from Dec 17-31.
I am expected to report to work to my new employer 17Dec 2012. Will this be allowed?
Will there be any possible sanction for doing so esp if I am not able to secure written permission from my old employer?

Your mentioned of offsetting of unused Annual Leave is to:

(i)                  Effect your last day of employment on 14 Dec 2012? Or
(ii)                Effect your last day of employment on 31 Dec 2012 (with leave clearance from 17 to 31 Dec 2012).

Please note to the difference and there would not be any implication for you to start a new
employment for scenario (i).
For scenario (ii), you could not start work with the new employer as you are still under the employment with the former employer.