By Philip Pullella

VATICAN CITY (Reuters) - Pope Francis on Tuesday made it simpler and swifter for Catholics to secure a marriage annulment, the most radical such reform for 250 years, and told bishops to be more welcoming to divorced couples.

Under the old norms, it often took years to win an annulment, with hefty legal fees attached. Francis said the procedure should be free and the new rules mean that a marriage might be declared null and void in just 45 days in some cases.

The announcement came the week after Francis signaled a more merciful approach to women who had obtained abortions and was another sign of his drive to shake up the hidebound Roman Catholic Church and try to soften some of its more rigid rules.

In a document known as a Motu Proprio, Latin for "by his own initiative", Francis reaffirmed traditional teaching on the "indissolubility of marriage", making clear that the Vatican was not in any form promoting or sanctioning divorce.

However, he said he would make it easier for separated couples to obtain an annulment -- a ruling whereby the Church decides that a marriage was not valid in the first place because certain prerequisites such as free will, psychological maturity and openness to having children were lacking.

Francis eliminated a previously mandatory review of an annulment decision by a second tribunal and gave bishops sweeping powers to judge quickly the most clear-cut cases.

He said he had decided to streamline procedures 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 CHANGE IN 250 YEARS

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

"The pope is seeking to respond pastorally to the tens of thousands of couples who are experiencing profound pain and alienation as a result of broken marriages," said Father James Bretzke, theology professor at Boston College.

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Francis took charge of the 1.2 billion member Church in 2013, replacing Pope Benedict, a theological hardliner well liked by conservatives for seeking to reaffirm traditional Catholic identity.

The Argentine pope has appeared a much more approachable figure and has spoken repeatedly of the need for the Church to show mercy and understand the needs of Catholics struggling to live by its rules.

Catholics who divorce and remarry in civil ceremonies are considered by the Church to be still married to their first spouse and living in a state of sin. This bars them from receiving sacraments such as communion.

While not changing this position, Francis wrote on Tuesday that bishops should show "particular pastoral concern" for divorced and remarried Catholics.

Many couples and priests have complained that the complex procedures discourage even those with legitimate grounds for an annulment from trying to obtain one.

Some 50,000 annulment procedures were launched last year, nearly half of them in the United States, according to the Center for Applied Research in the Apostolate at Georgetown University.

Francis is due to pay a landmark visit to the United States next month, where his progressive views on climate change and condemnation of rampant capitalism look certain to put him at loggerheads with Republican presidential hopefuls.

(Reporting by Philip Pullella; Editing by Crispian Balmer)