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A relentless Catholic Church campaign to derail a birth control law in the Philippines begins its final phase at the Supreme Court, with the verdict to have monumental impacts for millions of poor Filipinos.

The court on Tuesday will start hearing arguments to challenges against a family planning law that President Benigno Aquino, defying intense church pressure, helped steer through parliament late last year.

It is the last legal recourse for the Church, which had for more than a decade led resistance efforts to birth control legislation in the mainly Catholic Southeast Asian nation by lobbying and intimidating politicians.

The Church, which has threatened Aquino and other supporters of the law with excommunication, held prayer vigils and masses on Tuesday morning ahead of the court hearing in an effort to influence the judges.

"We ask the Holy Spirit to enlighten and inspire the lawyers who would be arguing for our position... and enlighten the justices of the Supreme Court," Bishop Gabriel Reyes told a mass in Manila held ahead of the opening of proceedings.

The law requires government health centres to hand out free condoms and birth control pills, benefiting tens of millions of the country's poor who would not otherwise be able to afford or have access to them.

In the Philippines, more than a quarter of the country's nearly 100 million people live on the equivalent of 62 cents a day, according to government data.

It also mandates that sex education be taught in schools and public health workers receive family planning training, while post-abortion medical care was legalised.

Proponents say the law will slow the country's population growth, which is one of the fastest in the world, and reduce the number of mothers dying at child birth.

"In the Philippines we lose 14 to 15 mothers a day due to pregnancy-related complications," international medical charity Merlin and its Philippine partner Likhaan said in a statement.

"But with this law, we are closer than ever before to being able to provide a low-cost solution which will directly save the lives of women and their babies, whose loss to their families and communities cannot be quantified."

The Supreme Court suspended the law in March so that the judges could hear the 15 formal petitions from a range of Church-backed groups arguing that it was unconstitutional.

The opponents argue the law erodes a section of the constitution that protects the "right of spouses to found a family in accordance with their religious convictions".

The Church wields strong influence in Philippines, a former Spanish colony where roughly 80 percent of the population remain Catholic.

Church leaders have helped lead two revolutions that toppled unpopular presidents in recent Philippine history, and continue to insist they have a right to influence the parliamentary and legal branches of government.

"When the rights of mother and child are endangered, when the family is being attacked, you can expect the Church to speak up," Catholic Bishops Conference of the Philippines head Bishop Socrates Villegas said on Monday.

However opinion surveys over many years have shown strong public support for birth control legislation.

And despite criticism from church leaders, Aquino enjoys near record-high popularity ratings half way through his six-year term.

Aquino has previously expressed optimism the court will rule in favour of the birth control law.

It was unclear on Tuesday how long the legal proceedings will take, or when the Supreme Court will make a decision. Court spokesman Ted Te told AFP no timeframe could be given.