The present invention relates to a temperature-stable labeling reagent of formula (0): in which: R1 represents H or an alkyl, aryl or substituted aryl group, R2 represents a detectable marker or at least two detectable markers interlinked by at least one multimeric structure, L is a linker arm comprising a linear chain of at least two covalent bonds and n is an integer equal to 0 or 1, R3 and R4 represent, independently of one another: H, NO2, Cl, Br, F, I, R2-(L)n-Y—X—, OR, SR, NR2, R, NHCOR, CONHR, COOR, —CO—NH—(CH2)3—(O—CH2—CH2)3—CH2—NH—R2, —CO—NH—(CH2)3—(O—CH2—CH2)4—CH2—NH—R2 with R=alkyl or aryl, A is a linker arm comprising at least one covalent double bond enabling the conjugation of the diazo function with the aromatic ring and u is an integer between 0 and 2, preferably 0 or 1, —Y—X— represents —CONH—, —NHCO—, —CH2O—, —CH2S—, —Z— represents —NH—, —NHCO—, —CONH— or —O—, m is an integer between 1 and 10, preferably between 1 and 3, and p is an integer between 1 and 10, preferably between 1 and 3. The present invention also describes a method for the synthesis of said labels and also applications for the labeling of biological molecules, in particular of nucleic acids, with a labeling reagent bearing the diazomethyl function.The invention is particularly suitable for use in the field of diagnostics.