  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.
