The fall from 33rd globally in 2001 to 62nd in 2010 and to 65th this year in the annual index comes despite a Scottish Government push, launched six years ago, to ensure 40% of board appointments were female.
The Counting Women In Coalition's Sex And Power 2014: Who Runs Britain? study, which produced the figures, concluded progress in women's representation had stalled or gone backwards - highlighting a 3.5% fall in local government leaders, to 13.1% in the past decade.
The coalition recommended all political parties take steps to increase the number of women candidates at all levels of elected office. It also said the media should ensure coverage of politics includes women and their views.
Report author Nan Sloane, director of the Centre for Women and Democracy, said: "If we really care about who has political power in this country we need to do something about the unrepresentative nature of our elected institutions.
"Along with other excluded groups women have already waited for generations for equal access to power, and we're still being asked to wait decades to achieve it. That's not good enough; we need real change now."
The government's diversity strategy aims to ensure 40% of board appointments are female.
While Holyrood does not have the power to set an enforceable quota for the number of women on public body boards it could seek a Section 30 order allowing powers to be transferred from Westminster. The SNP argues the process could be accelerated under independence by cutting out the need for Westminster approval.
In April, Cabinet Secretary for Equalities Shona Robison launched Women on Board, a consultation on a 40% quota for public boards such as NHS bodies and quangos. Ministers have also asked for views on whether future legislation for a 40% mandatory female membership on public bodies should be extended to the private sector.
Although women make up 52% of the population, only 36% of public board members and just 21% of board chairs in Scotland are women.
Norway was the first country in Europe to intorduce compulsory quotas in 2003, and threatened non-compliant companies with shutdown. Female membership of boards rose to 40% by 2009.
Katie Ghose, chief executive of the Electoral Reform Society, said: "At the tail-end of this Parliament, we are still not much closer to seeing real gender equality in our politics. Despite a headline-grabbing reshuffle earlier this year, the Prime Minister failed to reach his own target of making a third of his ministers female.
"With Britain slipping three places to 65th in the world ranking of parliaments by female representation this year, patience is wearing thin."
Dr Ruth Fox, director of the Hansard Society said: "If the political parties, the government and the media are serious about tackling this then the practical steps we have outlined could still be implemented in the coming months to improve the culture of politics and the general election campaign."