The Conservatives are balloting 1,333 senior members over plans to give Tory MPs the power to choose the successor to Michael Howard as leader.
Mr Howard is stepping down
The constitutional college is made up of MPs, MEPs, peers and senior party officials such as association chairmen.
Members have until 27 September to vote on the proposal, which would see party members merely consulted, rather than having the final say as at present.
At the moment MPs present a shortlist of two from which party members choose.
For the new proposals to go through, they have to be backed by two thirds of MPs and two thirds of the senior party officials who sit on the national convention.
The result of the vote on the rules will be known on 27 September. Under the new rules an election is thought likely to take about four weeks.
If the changes are not backed, the existing system for choosing a Conservative leader will be used - which would mean a result expected in the New Year.
See below for more details on the two systems:
The Existing Rules
Nominations for the leadership are presented to the chairman of the 1922 committee of backbench Conservative MPs. Each Tory MP then has one vote, and the candidate with the lowest number of votes drops out. There is then another round of voting, with the candidate getting the least number of votes dropping out. The process is continued until there are two candidates remaining. The entire membership of the Conservative Party then votes on the two candidates, with the one getting the most votes becoming leader.
The Proposed New Rules
Any MP receiving the nomination of 5% of Tory MPs (currently 10) can be declared a candidate for the party leadership by the 1922 Committee. The list of valid candidates is sent to the Conservative Party Board, which begins a consultation process:
* The Chairman of the National Convention will write to all association chairmen asking them to consult their members about their preferred candidate and providing guidelines on how they should do so. He will also consult area and regional officers.
* The Chairman of the Association of Conservative Peers, the leaders of the Conservative groups in the European Parliament, the Scottish Parliament, the Welsh Assembly and the Greater London Assembly and the Chairman of the Conservative Councillors Association will organise consultations amongst Conservative peers, MEPs, MSPs, Welsh Assembly members, Greater London Assembly members and councillors respectively.
The results of all these consultations will be published, giving the percentage of support for each candidate within each group. Conservative MPs will then elect the leader, in a format decided by the 1922 Committee. This is most likely to take the same format as previous leadership elections - a series of votes with the candidate polling the lowest number of votes eliminated at each stage. MPs will take account of the results of the consultation, but will not be bound by it. The whole process is expected to take about four weeks - though the more candidates there are the longer it will take.