Stress Corrosion Cracking (SCC) is often identified as one of the predominant failure causes of high-pressure gas transmission pipelines. Long-term operation of pipelines causes degradation of mechanical properties of pipeline steel, including a significant decrease in brittle fracture resistance and resistance to SCC. For oil and gas pipeline steels, there are regulatory requirements for impact toughness, disregarding steel condition, namely as-received or serviced. However, SCC resistance of pipeline steels, being very important for structural integrity, is not regulated. In the present work a new method to evaluate in-service degradation of pipeline steels, taking into account increasing susceptibility of operated metal to SCC, was developed. To ensure a safe operation of gas pipelines, it is suggested that the minimum allowable value of SCC resistance characteristics, in particular, the threshold of J-integral based stress intensity factor for SCC Jscc, should be defined and regulated. It is quite difficult to input these characteristics into regulatory requirements, therefore the developed method is based on modification of regulated limit values of impact toughness for as-received and serviced metal conditions separately, taking into account increasing susceptibility of serviced steel to SCC. The method was applied to evaluate in-service degradation of API 5L X52 ferrite-pearlite pipeline steels. The developed method considers different SCC susceptibility of as-received and serviced pipeline steels and shows there are limitations in regulating the same limit value of impact toughness for as-received and serviced pipeline steels.