Flight Crew departing WSSS is rerouted to the west for weather avoidance by dispatch prior to departure; but new flight plan is not filed. Crew received a 'cleared as filed' clearance from Clearance Delivery; which results in a track deviation.

Narrative

The flight departed with the wrong clearance. During flight; planning release 1 probably was L625 MEVIN B462 MYC V91.... this is just a guess; never saw it. Release 2 was generated for weather avoidance and that was what the crew saw in operations. It was WSSS M771 DULOP HCN G581 MYC V91.... this is a route that roughly parallels the first but west; thus avoiding some interesting weather. With no PDC available at WSSS; the clearance from delivery was VRM4B (the normal departure); Flight Plan Route. All seemed well. On departure; WSSS; ATC cleared Flight XX to LUSMO. We politely informed the Controller that was not on our route. We naively stated we were filed M771. We read our clearance to him; he copied and handed us off to the next Controller. About now; after yet another complete readback; we get the feeling that something is wrong and informed our Dispatch that something is not right with the clearance. We are soon apprised of the fact that Hong Kong will not accept us in their airspace and we are rerouted effectively back to the L625 routing. At the risk of being blunt; it appears that the Dispatcher forgot to file the new route with ATC. All we saw was release 2 and hadn't a clue what had transpired. In retrospect; a PDC would have saved the situation or a complete readback by Clearance Delivery. This is a 6-day trip with the aviators on a 12-hour body clock flip and not too many are totally on their game; but both pilots were as rested as could be expected. The fact that we were not on the 'normal' routing was clearly identified during planning but the weather avoidance route on the flight plan explained it.