Biaxially oriented, co-extruded polyester films comprising a base layer consisting of at least 90 wt. % thermoplastic polyester, preferably polyethylene terephthalate (PET), at least one sealable outer layer and a second non-sealable outer layer and, optionally, other intermediate layers, in addition to at least one flame-retardant agent, preferably organic phosphorous compounds. The inventive films are characterized by low inflammability, high UV stability, no embrittlement when subjected to thermal stress and a surface which is devoid of troublesome opacity. They are suitable for a multiplicity of uses both indoors and outside. The outer layers contain anti-blocking agents such as silicic acid having an average particle diameter of preferably less than 50 nm and/or 2 .mu.m. Preferably, the sealable outer layer consists of a copolyester which is made of ethylene terephthalate and ethylene isophthalate units.