The ritual washing of feet is a gesture of humility based on the belief that Jesus Christ washed the feet of his apostles on the evening of their final meal together before his arrest, crucifixion and death.

Vatican tradition has it that each year on Good Thursday the pope washes the feet of twelve priests of different ages and nationalities in a solemn ceremony held in Rome's St John Lateran's Basilica or in St Peter's Basilica.

The priests are seen as representing the apostles.

Though retired pontiff Benedict XVI revised the ritual in 2007 by selecting 12 lay men from Rome for the ceremony, the new Pope Francis shook up Catholic tradition Thursday by choosing to honour not only prisoners, but women as well as men.

Catholic traditionalists believe that all of Jesus's disciples were male and the historic decision to wash the feet of two girls -- one Italian Catholic and one of Serbian Muslim origin -- is likely to create friction in some circles.

The gesture appears to open to interpretation the importance of the gender of the apostles.

Calls from groups within the Church for women to be ordained have been rebutted on the basis that Jesus's apostles were men.

There is no Catholic dogma which explicitly forbids the participation of women, though a circular letter from 1988 which explains the importance of the tradition refers specifically to the "washing of the feet of chosen men."

Cardinal Jorge Mario Bergoglio -- as Pope Francis was previously known -- had already washed and kissed the feet of women in past ceremonies in Argentinian jails, hospitals and old people's homes, including pregnant mothers and AIDS patients.

Other Catholic priests and bishops have also included women in the feet washing although this is considered highly unusual.

The role of women in the Church has become an increasingly hot topic, with campaigners and rebel clerics calling for a review of the dogma which bans female priests -- particularly as male priest numbers are dropping in the West.

The practice of washing feet appears to stem from ancient hospitality customs, whereby hosts would provide servants to clean the feet of newly arrived guests.

There are some variations to the ritual as performed in churches across the world today.

In some cases whole congregations participate in the washing, or take it in turns to wash each other's feet.