Margaret Thatcher was granted honorary membership in 1975
The Carlton Club, the elite private members' club for Conservatives, is to allow women to become full members - 176 years after it was founded.
Former PM Lady Thatcher had been the sole full female member, having been granted "honorary" membership in 1975.
Other women were limited to "associate member" status without voting rights and excluded from some areas.
Party leader David Cameron and former leader Iain Duncan Smith had declined membership because of the policy.
There have been several previous attempts to grant women equal status.
Despite a majority backing the move in a vote last year, it was defeated because, under the club's rules a change to its constitution had to be supported by two thirds of those who voted.
But Mary Sharp, an assistant secretary at the club, said a motion to delete rule 2c, which said women could be "associate members" but could not "otherwise be members of the club", had been passed at Tuesday's AGM, although she would not divulge the exact split in votes.
She said a general committee meeting next week would look at "the ramifications" but the new rules would be "virtually immediate".
"There are just a few housekeeping issues we have got to deal with which won't happen overnight," she added.
Members present at Tuesday's meeting included former leader Michael Howard and shadow defence secretary Liam Fox.
The Carlton Club, which describes itself as "the oldest, most elite and most important of all Conservative clubs", is only open to "persons of full age who support the Conservative Party".
No 'no-go areas'
Dress code during the week is "jacket and tie for gentlemen" while "ladies may wear trousers, but not denims".
Membership has been automatic for every Conservative leader since it was formed in 1832. But its policy towards women has prompted some to turn it down.
The Carlton Club describes itself as "the most elite" Conservative club
Explaining his decision not to accept in 2001, Mr Duncan Smith told the Spectator: "I don't care what private clubs do. That's for them to decide and I'm not going to interfere in that.
"But this invitation comes to me because of what I am, rather than to me personally; as the leader of a Conservative Party that believes that there should be no 'no-go' areas for women."
Previously women members had been able to use all facilities except for the members' bar on the ground floor and the members' dining room at lunch time.
They also could not attend or vote at general meetings.
The Conservative Party declined to comment on the club's change in rules.
The party's spokeswoman for women, Theresa May told the Daily Mail: "I'm just sorry that it took them this long to join the 21st Century."