The number of female MPs is likely to fall at the next election without measures such as all-women shortlists, a report says. The Hansard Society called on all the parties to use "equality guarantees" to ensure more equal representation.
Of the major parties, only Labour has adopted a change in the law in 2002 to allow all-women lists for candidates.
Both contenders for the Conservative leadership, David Davis and David Cameron, have ruled this course out.
'Justice and fairness'
The report, Women at the Top 2005: Changing Numbers, Changing Politics?, says women in Parliament can have a "feminising" effect on legislation.
It includes case studies suggesting there are gender differences in MPs' attitudes, concerns and perspectives.
Equal representation of men and women is "vital for simple reasons of justice and fairness", it adds.
Sarah Childs, the joint author of the report, said: "Unless all parties use equality guarantees, such as all-women shortlists, it is most unlikely that they will select women in those vacant seats."
Following the last general election, Labour has 98 women MPs, the Conservatives 17, Liberal Democrats 10 and Northern Ireland parties three.
In total, women make up 128 - just under 20% - of MPs.
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