The award of the 2013 Nobel Prize for Physics acknowledged the leading role of Europe in particle physics, which has a global community of over 10,000 scientists. To reinforce its pole position throughout the 21st century, Europe must be ready to propose an ambitious post-LHC accelerator project by 2018/19. This is one of the main recommendations of the updated European Strategy for Particle Physics, adopted by the CERN Council in May 2013.The EuroCirCol conceptual design study is a direct response to this recommendation, initiating a study for a 100 TeV energy-frontier circular collider through a collaboration of institutes and universities worldwide.A new research infrastructure of such scale depends on the feasibility of key technologies pushed beyond current state of the art. Innovative designs for accelerator magnets to achieve high-quality fields up to 16 T and for a cryogenic beam vacuum system to cope with unprecedented synchrotron light power are required. The effects of colliding two 50 TeV beams must be mastered to meet the physics research requirements. Advanced energy efficiency, reliability and cost effectiveness are key factors to build and operate such an accelerator within realistic time scale and cost.This proposal is part of the Future Circular Collider study under European leadership, federating resources worldwide to assess the merits of different post-LHC accelerator scenarios. It forms the core of a globally coordinated strategy of converging activities, involving participants from the ERA and beyond. Organisations joining this study from Japan and the USA are expected to take part in a global implementation project and a suitable governance model will be drawn-up accordingly.The main outcome of EuroCirCol will be laying the foundation of subsequent infrastructure development actions that will strengthen the ERA as a focal point of global research cooperation and as a leader in frontier knowledge and technologies over the next decades.