The Vatican on Monday said all of the 133 Catholic cardinals who will vote to choose a successor to Pope Francis are now in Rome ahead of the conclave starting this week.

According to the Vatican, the cardinals will gather in secret in the Sistine Chapel starting at 4:30 pm (1430 GMT) on Wednesday.

They will keep voting until one choice among them has a two-thirds majority, and becomes head of the world’s 1.4 billion Catholics.

The conclave itself begins Wednesday morning with the celebration of a special Mass in St. Peter’s Basilica.

In the afternoon, the cardinals will formally process into the Sistine Chapel, the storied 15th century worship space adorned with frescoes by Michelangelo, where they will begin voting for the next pope.

They are expected to take one vote on Wednesday afternoon. Subsequent days will have two votes each morning and afternoon. It takes a two-thirds majority for someone to be elected.

According to conclave regulations, if no-one has been chosen after the first three days, the cardinals should take a day-long “pause of prayer” before continuing.

The only signal given to the outside world about the deliberations will come from a chimney installed above the chapel. The cardinals will burn their ballots, adding a chemical product to create one of two colours of smoke: black for an inconclusive vote; white when there is a new pope.