This paper presents a comparative investigation of the early phase of combustion initiated by a focused pulsed laser beam and a conventional spark-discharge. The comparative approach is applied to the characterization of combustible mixtures of natural gas and biogas with varying CO2 content. Interferometry and Schlieren imaging are used to probe flame kernel formation and its subsequent transition to a self-sustained flame. The pressure rise in the chamber is also recorded by means of a fast-response piezo-electric pressure transducer. From these diagnostics, differences between the early phases of laser-induced and sparkignition are revealed. Unlike in laser-induced ignition, sparkignition features a delay in the transition from the early flame kernel to a self-sustained flame (attributed to heat loss), which leads to failed ignition for mixtures with high CO2 content. The role of CO2 as an ignition retardant is observed by comparing fuel/air mixtures with and without CO2.