As Joint Chairman (alongside Liberal Democrat MP Robert Maclennan) of the Labour-Liberal Democrat Joint Consultative Committee on Constitutional Reform, Cook brokered the 'Cook-Maclennan Agreement' that laid the basis for the fundamental reshaping of the British constitution outlined in Labour's 1997 general election manifesto. This led to legislation for major reforms including Scottish and Welsh devolution, the Human Rights Act and removing the majority of hereditary peers from the House of Lords. Other measures have not been enacted so far, such as further House of Lords reform. On 5 May 2011 the United Kingdom held a referendum on replacing the first-past-the-post voting system with the Alternative Vote method. On 6 May it was announced that the proposed move to the AV voting system had been rejected by a margin of 67.9% to 32.1%.