Pope Francis on Tuesday made it easier, quicker and free for Catholics to have their marriages annulled in the biggest reform of the Church's approach to marital breakdown in nearly three centuries.

Details of the changes were unveiled Tuesday with the publication of a letter from Francis to Catholic churches across the world.

In it, the Argentinian pontiff says annulments will henceforth require only one decision rather than having to be approved by two church tribunals, as currently.

A streamlined procedure is to be introduced with most cases to be handled by individual bishops rather than being subject to a hearings process.

Appeals to a Vatican court against individual annulments will still be possible but only in exceptional cases.

The Pope's letter follows a year-long review by Vatican experts. It also asks bishops' conferences to ensure that, wherever possible, here are no costs involved in the process of securing an annulment.

- A friendlier Church -

The proposed reform is the latest in a series of steps initiated by Francis with the aim of making the Church appear more compassionate in relation to believers in difficult situations with regard to their faith.

Last week he made it easier for women to be granted absolution for having had abortions and he has also signalled a new, less judgemental attitude to homosexuality.

Pio Vito Pinto, the cardinal who headed the commission on annulments, told reporters the changes were the most significant since the mid-18th Century.

While Francis is seeking to democratise the procedure in a way that would appear to make an increase in the number of annulments likely, his letter does not amend the exceptional conditions under which they can be granted, Pinto said, playing down suggestions the reform amounted to a disguised introduction of Church-approved divorce.

Francis strongly reaffirms the principle of the indissolubility of marriage while highlighting the "enormous number of believers" for whom annulment is currently not an option for various reasons.

Although the notion of marriage being for life is a fundamental tenet of the Catholic faith, divorce has become commonplace among believers in developed countries.

Church doctrine allows for unions to be cancelled -- effectively declared to have never existed -- when the marriage is judged to have been fundamentally flawed from the outset.

Possible justifications for reaching this conclusion include non-consummation of the marriage, one or both partners having entered into it without the intention of staying in the relationship, or one of the partners having no desire to have children.

- Kings and Princesses -

Access to the annulment procedure currently varies widely.

There is virtually no provision for it in many dioceses in the developing world while many ordinary Catholics in wealthier countries do not understand the complex procedures or cannot afford legal guidance.

For centuries there has been a perception that annulments are more easily obtained by the wealthy and powerful.

England's King Henry VIII's failure to secure a Vatican annulment of one of his marriages led to the creation of the Church of England in the 16th Century.

In one of the most high-profile recent cases, Princess Caroline of Monaco obtained the annulment of her first marriage, to Frenchman Philippe Junot, in 1992, leaving her free to remarry in the Church.

There was also controversy in 2006 when Australian actress Nicole Kidman married Keith Urban in a Sydney church following her divorce from Tom Cruise.

Kidman was reported at the time to have secured an annulment but it later emerged that the Church in Australia had simply confirmed that it did not recognise her first marriage because it had been conducted in Cruise's Church of Scientology.

Without an annulment, a Catholic who divorces and remarries is deemed to be living in sin and is unable to take communion.

Critics say this exclusion of the divorced from the Church's holiest sacrament is particularly cruel. Why, they argue, should a murderer who confesses his sins be able to take communion while a woman who seeks a divorce to escape a violent relationship cannot.

The status of divorcees is among questions being considered as part of an ongoing review of Catholic teaching on the family.

Bishops will seek to reach a consensus on this and other vexed issues at a synod in October, after which Francis will announce what, if any, reforms will be made.