Pope Benedict XVI this week overturned papal election rules set by John Paul II in 1996, which allowed for a possible simple majority vote, and restored the long-standing custom that a two-thirds majority (or consensus) is always necessary to elect the Bishop of Rome.

But in returning to the older voting system, the Pope not only abrogated the very procedure that elected him in 2005, he also added his own innovation. In a motu proprio issued on Tuesday, he said that if no one is elected pope after 13 days (some 34 ballots) the vote should be narrowed exclusively to the top two candidates. However, a two-thirds majority would still be required for election.

One Vatican canon lawyer told The Tablet that the return to the once-unbroken custom was long overdue. "Papa Wojtyla's voting rules could have caused a schism," the canonist said.

Pope John Paul II in his 1996 document on procedures regulating the papal election - Universi Dominici gregis - allowed for a simple majority if no candidate had received two-thirds of the vote after 13 days. But canon lawyers have since argued that such legislation could actually affect the conclave from the outset. "Whenever a candidate receives more than half of the votes, or even when he comes close to it, and his supporters are perceived by the minority as determined, the election is virtually over," said Fr Ladislaw Orsy SJ, a canon law expert. He said even early in the process there would be "no point in waiting", whereas the older custom means that "a group that could master absolute majority, but no more, has to be ready for a compromise".

Fr Orsy told The Tablet that Pope John Paul's legislation allowed for a "bare majority to overcome the others too easily". The Second Vatican Council stressed that the papal office was a principle of unity and, therefore, "the Bishop of Rome should not assume his office with a community more or less evenly divided".

Fr Tom Reese, editor of the book Inside the Vatican, said that Pope Benedict was to be "congratulated for returning to the two-thirds tradition", but he said having a run-off between the top two candidates was a "mistake". "One wonders what will happen if neither of the top two candidates can get a two-thirds majority in the subsequent voting. We could have a deadlocked conclave with no possibility of voting for a compromise candidate," he said.

Some have theorised that Pope John Paul II may not have been elected under the 1996 legislation, pointing out that the ultraconservative Cardinal Giuseppe Siri of Genoa was believed to have garnered a simple majority by the fourth ballot. By contrast, the 1996 electoral procedures seem to have favoured Pope Benedict XVI's election. It is thought that he initially garnered a little over one-third of the votes - something that, under the rules he re-established this week, might have led his supporters to look for another candidate.