The Spaak Report concluded that a sector-by-sector integration of the European economies would be difficult. Instead, a horizontal integration of the economy, by the gradual elimination of trade barriers, seemed to be the way to continue. The goal was to be achieved by creating a customs union.
On the integration of the energy sectors, there was a different stance for nuclear energy and for hydrocarbon energy sources (oil, coal). The integration of the European nuclear energy sector was desirable because of the costs involved, which surpassed the financial capacity of individual states. The integration of the development of nuclear energy at a supranational level meant more efficient cost sharing for the development of nuclear energy. The integration of hydrocarbon energy sources at a supranational level was less feasible, as these energy sources were managed mainly by multinational companies. The integration of electricity and fuel gas seemed irrelevant, as they were distributed solely at a national level.