We have surveyed a sample of massive star-forming regions located over a range of distances from the Galactic center for methyl formate, HCOOCH_3_, and its isotopologues H^13^COOCH_3_ and HCOO^13^CH_3_. The observations were carried out with the APEX telescope in the frequency range 283.4-287.4GHz. Based on the APEX observations, we report tentative detections of the ^13^C-methyl formate isotopologue HCOO^13^CH_3_toward the following four massive star-forming regions: Sgr B2(N-LMH), NGC 6334 IRS 1, W51 e2, and G19.61-0.23. In addition, we have used the 1mm ALMA science verification observations of Orion-KL and confirm the detection of the ^13^C-methyl formate species in Orion-KL and image its spatial distribution. Our analysis shows that the ^12^C/^13^C isotope ratio in methyl formate toward the Orion-KL Compact Ridge and Hot Core-SW components (68.4+/-10.1 and 71.4+/-7.8, respectively) are, for both the ^13^C-methyl formate isotopologues, commensurate with the average ^12^C/^13^C ratio of CO derived toward Orion-KL. Likewise, regarding the other sources, our results are consistent with the ^12^C/^13^C in CO. We also report the spectroscopic characterization, which includes a complete partition function, of the complex H^13^COOCH_3_ and HCOO^13^CH_3_species. New spectroscopic data for both isotopomers H^13^COOCH_3_and HCOO^13^CH_3_, presented in this study, have made it possible to measure this fundamentally important isotope ratio in a large organic molecule for the first time.