The main goal of this contribution is to evaluate the possible impacts of ambitious climate mitigation policies on future European energy security. We consider four dimensions of energy security: availability of energy sources, energy affordability, electricity and sustainability. In total, we use 13 indicators to evaluate these four dimensions. Relying on the POLES model, five prospective scenarios have been built: a reference scenario consistent with the NDCs in 2030 and without enhanced ambitions, two 2°C scenarios distinguished by the start date of enhanced climate policy, and two 1.5°C scenarios representing different levels of final energy demand. These scenarios provide highly detailed data regarding energy systems. They are used to derive energy security indicators at EU and national level. At the aggregated EU level, our analysis shows that ambitious climate policies improve the level of energy security dimensions related to the share of residential energy expenditure in GDP, total (households and economic sectors) energy costs in GDP, and energy intensity of GDP. Nevertheless, these positive effects must be qualified for the Central and Eastern European countries in particular, which appear to be much more vulnerable than the EU15 countries. This work also reveals points of vigilance that must be taken into account. The first weakness concerns security of natural gas supply since Russia is by far the main supplier for a number of countries, including in the most ambitious mitigation scenarios, even if quantities imported are much lower. A second element concerns grid stability issues in light of the decarbonisation of the electricity mix and the required rise in intermittent renewables. The country-level scenario analysis was also supplemented by a country case study for Bulgaria and Poland. The focus on these countries is critical, as energy security is a key socioeconomic objective closely linked with climate and energy policy in the CEE region.