VATICAN CITY (Reuters) - Pope Francis, making the most substantial changes to Catholic marriage annulment procedures in centuries, on Tuesday radically simplified them and said bishops should give greater help to divorced couples.

In a move that again showed his desire for the Church to bemore merciful to Catholics in difficulty, Francis reaffirmed traditional teaching on the "indissolubility of marriage", but streamlined annulment procedures many considered cumbersome, lengthy, outdated and expensive. An annulment, formally known as a "decree of nullity", is a ruling that a marriage was not valid according to Church law because certain prerequisites, such as free will, psychological maturity and openness to having children, were lacking.

Francis said the procedures needed to be speeded up so that Catholics who sought annulments should not be "long oppressed by darkness of doubt" over whether they could have their marriages declared null and void.

Most annulments are granted at a local level and only the most complicated cases reach a special court at the Vatican, known as the Rota. Francis said the procedures, which can cost thousands of dollars in legal fees, should be free.

Monsignor Pio Vito Pinto, dean of the Vatican court that rules on annulments, told a news conference the new rules were the most substantive changes to annulment laws since the papacy of Benedict XIV, who reigned from 1740 to 1758.

The reform was keenly awaited by many couples around the world who have divorced and remarried outside the Church. The 1.2 billion member Church does not recognise divorce and Catholics who re-marry in civil ceremonies are considered to be still married to their first spouse and living in a state of sin. This bars them from receiving sacraments such as communion.

Many couples and priests have complained that the current procedures are outdated and too complicated, and discourage even those with legitimate grounds for an annulment from trying.