Can a simple voltage divider circuit be used to accurately measure voltage?
The divider has an output impedance of 2.5M ohm, so it should not be connected directly to the ADC or you'll get considerable error. 

Why not use 10K + 10K 1%? 

Edit: According to the manual you linked in your comments, the maximum source impedance allowable for the board you have in mind (it appears to use a PIC) is 10K ohms. 



So 10K + 10K is fine, and you could use a higher value if you like (as much as 20K+20K if you always have the battery connected) or stick with the 10K+10K if you might put a switch in there to reduce drain on the battery.