Terminally ill children are set to win the right to die in Belgium when lawmakers vote Thursday to extend a decade-old law allowing euthanasia for adults after months of often heated public debate.

Despite strong opposition from the Church and some pediatricians, the legislation is widely expected to pass, making the largely Catholic country the world's second after the Netherlands to allow mercy-killing for children.

But unlike the Dutch across the border, where euthanasia is allowed for children over 12, the draft bill before Belgium's House of Representatives lifts all age restrictions on the right of the incurably sick to end their lives -- a fact that has caused considerable controversy.

Socialist senator Philippe Mahoux, the author of the country's ground-breaking 2002 "right to die" legislation and himself a doctor, called for the law to be widened to minors because medics were helping children in pain die as a question of mercy, but illegally.

Euthanasia is "the ultimate gesture of humanity" and "not a scandal", he said. "The scandal is illness and the death of children from disease."

The draft bill states that a child must be equipped "with a capacity of discernment and be conscious" on requesting to die.

The child must also "be in a hopeless medical situation of constant and unbearable suffering that cannot be eased and which will cause death in the short-term".

Counselling by doctors and a psychiatrist or psychologist is required, as is approval by the parents.

Before a first vote in the Senate in December, where the proposal was passed with a huge majority, the upper house consulted dozens of medical specialists, lawyers and interest groups.

But in the months of debate leading up to the vote, religious leaders of all faiths argued that extending euthanasia to the young undermined moral values and risked "trivialising" death.

The Catholic Church has staged "a day of fasting and prayer" in protest and this week some 160 pediatricians petitioned lawmakers to postpone the vote on the grounds it was both ill-prepared and unnecessary.

Euthanasia 'not a happy end'

"Pain can be eased nowadays, there's been huge progress in palliative care," said cancer specialist and signatory Nadine Francotte.

In a stormy exchange in parliament on the eve of the vote, a Christian Democrat MP opposed the proposal on the grounds that modern medication could relieve pain in very sick youngsters allowing illness to run a natural course to death.

"Euthanasia is not the only way to die in dignity," said parliamentarian Sonja Becq. "Euthanasia is not 'a happy end'".

But Brussels palliative specialist Dominique Lossignol said it was mistaken to believe palliative care could remedy suffering.

"We do not have control over all types of pain, either physical or moral," he told AFP. "We doctors have been asking for an extension of the law for years."

Thursday's vote in the lower house is expected to see Socialists, Liberals and Greens members line up in favour, with centrist Christian-leaning parties opposed.

Critics say the legislation fails to address problems and potential loopholes such as possible discord between two parents over a child's request to be euthanised.

And how can adults be sure that a child has the "discernment" necessary to decide to give up life or even really understands the notion?

"Experience shows us that in cases of serious illness and imminent death, minors develop very quickly a great maturity, to the point where they are often better able to reflect and express themselves on life than healthy people," a group of pediatricians in favour of the legislation said in December.

Polls show a majority of Belgians backing the proposal but there was a surge of concern last year when a 44-year-old in distress after a failed sex change was euthanised on psychological grounds in a highly-publicised case.

Belgium registered 1,432 cases of euthanasia in 2012, up 25 percent. They represented two percent of all deaths.