We have surveyed a sample of massive star-forming regions located over a range of distances from the Galactic centre 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--287.4~GHz. Based on the APEX observations, we report tentative detections of the \(^{13}\)C-methyl formate isotopologue HCOO\(^{13}\)CH\(_{3}\) towards 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 1~mm 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 Orion-KL Compact Ridge and Hot Core-SW components (68.4\(\pm\)10.1 and 71.4\(\pm\)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, has made it possible to measure this fundamentally important isotope ratio in a large organic molecule for the first time.