By Nick Assinder
BBC News political correspondent
The Conservative leadership race has been thrown wide open, if not into crisis, after the vote to leave the election of Michael Howard's successor to grass roots members.
Mr Howard is expected to step down in October
It is the nightmare Mr Howard and many party bosses had dreaded, believing it will lead to weeks, if not months of internal wrangling.
Worse, they fear they may end up with a leader who - while acceptable to ordinary party members - is opposed by a large section of the parliamentary party and, therefore, unable to exert any control over his or her MPs.
The decision must raise the prospect that other candidates who have been hanging back will now throw their hats into the ring, opening up the prospect of a bitter contest.
It is difficult to say definitively how the rule change will boost or damage the prospects of each contender.
Recent polls suggest the notion that either system would automatically favour Ken Clarke and David Davis - the two leading candidates - has been thrown into doubt.
More MPs are publicly backing Mr Davis than any other candidate - but many of them have yet to declare their support for anybody.
Some MPs will undoubtedly see the result as a victory for democracy, with it now being left to the good sense of ordinary party members to select a formidable new leader.
RULES BALLOT RESULT
Overall: 61% in favour of changing rules
MPs: 71% backed the proposal, 26% against
Voluntary activists: 58% wanted rule change, 42% against
Peers/MEPs: 63% wanted rule change, 37% against
But it seems likely that the Tories will be pitched into a prolonged period of limbo at a time when many in the party believe it should be attacking a vulnerable government.
What has given party bosses the most sleepless nights is the notion that they could land themselves with a re-run of the 2001 contest which ended with the shock election of Iain Duncan Smith.
That is one Groundhog Day that many in the party leadership seem eager not to live through.
Firstly, that contest dragged on for weeks, ensuring internal party rows could not be closed off.
And for much of that period, media focus on the Tory party was concentrated almost entirely on the contest and splits.
And, of course, it saw major party figures with a high public profile, like Ken Clarke and Michael Portillo, rejected in favour of Mr Duncan Smith who, because of the way he was elected, never managed to unite MPs behind him.
His entire, brief period in office was then overshadowed by internal wrangling over the direction of the party and his own effectiveness.
It all came to an unsightly head at the party's conference in 2003, which was a hotbed of plotting against Mr Duncan Smith with rival camps almost literally setting up their stalls in Blackpool hotels.
At the last minute, Mr Howard emerged as the surprise "unity" candidate and was elected without a contest and an audible sigh of relief - the way the Tories always used to do it.
It was that nightmare period which persuaded Mr Howard that the current system could not continue and that MPs should be given the decisive say.
That would, he believed, ensure the contest would be short, if not sweet, and at least end up with a leader MPs would have chosen themselves and have to rally behind - in so far as Tory backbenchers are capable of doing that.
It was also argued that MPs have a far better knowledge of their colleagues and who would be most likely to lead the party to election victory.
Needless to say, some MPs and many constituency parties believe backbenchers are unrepresentative of the wider party and, as a result, not capable of choosing a leader able to re-engage with the grassroots.
Thanks to the latest vote, Mr Howard now faces the prospect of having to remain in office until, quite likely, the New Year and preside over another potentially divisive contest.