About one in five MPs is a woman
An exodus of MPs at the next election has provided a one-off chance to get more women and black people into the Commons, a special MPs committee says.
But Britain's political leaders must act now or - they may not get another chance for 20 years, the report adds.
Some 89 MPs have said they are standing down so far, many as a result of the expenses scandal.
The report calls for half of them to be replaced by women and a big increase in black, Asian, disabled and gay MPs.
UK democracy is under threat unless Parliament becomes more representative of the country as a whole, the cross-party panel says.
The committee, known as a Speaker's Conference, says MPs should be more representative of the UK as a whole.
It recommends replacing most of the MPs standing down at the next election with women and increasing the number of black, Asian and disabled MPs.
It stops short of proposing quotas or all-women shortlists, calling instead on the party leaders to demonstrate their commitment to diversity in the selection of candidates for the next general election.
It says the parties should put more candidates from under-represented groups in front of local constituency selection panels - and urges them to report back on progress by 12 October.
It says: "This is an important window of opportunity. If a more diverse group of candidates is not selected in these seats now the incumbency factor makes it likely that many of the constituencies concerned will not see another opportunity for change for the next fifteen to twenty years".
Gordon Brown was asked about the report at prime minister's questions, saying there was "an important opportunity to further help increase the number of women, disabled and black, Asian and minority ethnic people in our Parliament".
"The government is committed to ensuring greater diversity of representation in public and political life," he told MPs when Labour MP Anne Begg, the panel's vice chair who is herself disabled, asked him to commit the government to the interim proposals and urge other party leaders to do the same.
As Tory backbenchers shouted out: "What about northerners?", Mr Brown urged the Conservatives to "think" after their opposition to the Equality Bill.
Earlier this week ministers denied reports that they were to introduce quotas for the number of northerners in quangos.
But the Government's Equalities Office did say they wanted changes to avoid public bodies' boards being too London-centric.
The call for parties to make sure 50% or more of new candidates were female - and to secure an increase in the numbers from ethnic minorities or with disabilities - emerged from a cross-party review set up by former Commons speaker Michael Martin on how to make the chamber more representative.
The Speaker's Conference, the first to be convened for 30 years, was tasked with finding ways to boost the numbers of women, disabled people, gay people and lesbians and members of the ethnic minorities elected to Parliament.
The report said trust in MPs had been further damaged by the MP expenses scandal and was now "a matter of urgent concern".
"There is a real danger that in this furore people have lost sight of the importance of democracy," it warns.
It said an important way to boost the legitimacy and effectiveness of Parliament was to make sure it better reflected British society.
But the unusually-high number of MPs standing down at the next election - 89 so far or 13.8% - presented a once in a generation opportunity to ensure Parliament more closely resembled the UK population as a whole.
Anne Begg said: "The choices local and national political parties make as to the candidates who will represent them at the next election will be central to shaping what the next House of Commons looks like, and the extent to which it is genuinely changed.
"Increasing the diversity of MPs - having more women MPs, more black and minority ethnic MPs and disabled MPs, who can speak in our debates with the authority of lived experience - would make the House of Commons a more legitimate, credible and effective legislature.
"For a local party to select a capable candidate from an under-represented group is a really practical way for that party to demonstrate its support for political and Parliamentary reform."
The report calls on the parties to monitor the gender, ethnicity, disabilities and sexual orientation of candidates in the selection process.
It also urges the parties to appoint a named party officer for monitoring the access requirements of disabled people.