A flame retardant composition containing a compound of Formula I: (A-H) a (+) [ME b m+ (H 2 PO 4) x1 (-) (HPO 4) x2 2(-) (PO 4) x3 3(-) (PO 3) y (-)] (a) · pH 2 O; and a derivative of 9,10-dihydro-9-oxa-10-phosphorylphenanthrene-10-oxide or polymeric complexes of this derivative, according to at least one of the Formulae IIa, IIb, IIc and IId: wherein each ME is independently one of Cu, Mg, Ca, Zn, Mn, Fe, Co, Ni, TiO, ZrO, VO, B, Si, Al, Sb, La, Ti, Zr, Ce or Sn and each M is independently one of a metal of the second, third, twelfth or thirteenth group of the periodic system; wherein x = 2 or 3; wherein n ≥ 1; wherein (A-H) (+) is a residue of Formula III; wherein X is one of H, CN, C NH 2, C NH 2, C NHCN, or a condensation product of one or more of H, CN, C NH 2, C NH 2, C NHCN; wherein m is an integer in the interval [1; 4]; wherein a is an integer in the interval [1; 6]; wherein b is an integer in the interval [1; 14]; wherein each of x1, x2, x3, y is an integer in the interval [0, 12], wherein at least one of x1, x2, x3, y is > 0; wherein p is an integer in the interval [0; 5]; wherein the following applies: a + m b = x1 + 2 x2 + 3 x3 + y; wherein the compound of Formula I contains at least one of the following compounds or consists of at least one of the following compounds: melamine (aluminium phosphate) (Mel-H) 2 (+) [AlP 3 O 10] 2(-); melamine (zinc phosphate) [(Mel-H) +] 2 [ZnP 2 0 7] 2-; melamine (magnesium phosphate) (Mel-H) x + [Mg 2+] 2 [PO 3 -] (x+1) [PO 4 3-], wherein x is between 1 and 4, in particular (Mel-H) 2 (+) [Mg 2 P 4 O 13] 2(-); or oligomers or polymers thereof, wherein the weight ratio between the compound of Formula I and the derivative of 9,10-dihydro-9-oxa-10-phosphorylphenanthrene-10-oxide or its polymeric complex is between 1:30 and 30:1. a compound of Formula I: (A-H) a (+) [ME b m+ (H 2 PO 4) x1 (-) (HPO 4) x2 2(-) (PO 4) x3 3(-) (PO 3) y (-)] (a) · pH 2 O; and a derivative of 9,10-dihydro-9-oxa-10-phosphorylphenanthrene-10-oxide or polymeric complexes of this derivative, according to at least one of the Formulae IIa, IIb, IIc and IId: wherein each ME is independently one of Cu, Mg, Ca, Zn, Mn, Fe, Co, Ni, TiO, ZrO, VO, B, Si, Al, Sb, La, Ti, Zr, Ce or Sn and each M is independently one of a metal of the second, third, twelfth or thirteenth group of the periodic system; wherein x = 2 or 3; wherein n ≥ 1; wherein (A-H) (+) is a residue of Formula III; wherein X is one of H, CN, C NH 2, C NH 2, C NHCN, or a condensation product of one or more of H, CN, C NH 2, C NH 2, C NHCN; wherein m is an integer in the interval [1; 4]; wherein a is an integer in the interval [1; 6]; wherein b is an integer in the interval [1; 14]; wherein each of x1, x2, x3, y is an integer in the interval [0, 12], wherein at least one of x1, x2, x3, y is > 0; wherein p is an integer in the interval [0; 5]; wherein the following applies: a + m b = x1 + 2 x2 + 3 x3 + y; wherein the compound of Formula I contains at least one of the following compounds or consists of at least one of the following compounds: melamine (aluminium phosphate) (Mel-H) 2 (+) [AlP 3 O 10] 2(-); melamine (zinc phosphate) [(Mel-H) +] 2 [ZnP 2 0 7] 2-; melamine (magnesium phosphate) (Mel-H) x + [Mg 2+] 2 [PO 3 -] (x+1) [PO 4 3-], wherein x is between 1 and 4, in particular (Mel-H) 2 (+) [Mg 2 P 4 O 13] 2(-); or oligomers or polymers thereof, wherein the weight ratio between the compound of Formula I and the derivative of 9,10-dihydro-9-oxa-10-phosphorylphenanthrene-10-oxide or its polymeric complex is between 1:30 and 30:1.