Although the European Union has legislated in the area of energy policy for many years, the concept of introducing a mandatory and comprehensive European Union energy policy was only approved at the meeting of the informal European Council on 27 October 2005 at Hampton Court. The EU Treaty of Lisbon of 2007 legally includes solidarity in matters of energy supply and changes to the energy policy within the EU. Prior to the Treaty of Lisbon, EU energy legislation has been based on the EU authority in the area of the common market and environment. However, in practice many policy competencies in relation to energy remain at national member state level, and progress in policy at European level requires voluntary cooperation by members states.