Pope Francis has decried the number of Catholics now cohabiting and said he would order a study to tackle the question.

"Today, many Catholics don't get married but live together. Marriage is provisional. This is a serious problem," the pontiff told members of the Vatican's Synod of Bishops.

"In October, we need to commission a pastoral study of the family," Francis said.

He did not elaborate who would carry out the study.

The Synod of Bishops was set up by Pope Paul VI in 1965 to study specific issues in the Catholic Church and put forward solutions.