According to the BBC at the time of David Cameron's election there were 253,600 Tory members.
For the 2000 ballot - during William Hague's leadership - membership equalled 302,443.
* Based on participation in the Built to Last referendum.
4pm update: CCHQ has just issued this statement: “There is a three month eligibility rule to vote in ballots, so the large number of new members who have joined since our local election success in May will not have had the opportunity to vote. In addition, the Conservative Party has a decentralised membership system, and we rely on constituency associations to provide updated membership data to the centre. Associations are most active at updating their membership records in September to ensure that their membership data is able to be transferred on to the new register of electors which is published at the end of the year. Registered Party membership is therefore higher in the autumn than it is during the summer. Conservatives continue to have more members than the Labour Party and Liberal Democrats combined.”