Belgium is expected on Thursday to become the first country in the world to abolish all age restrictions on the right to die, extending permitted euthanasia to terminally ill children.

Following months of painful and divisive debate the Belgian parliament is expected to pass a law extending euthanasia – which has been possible in the country for the past 12 years – to minors.

Belgium is one of only three countries to have legalised euthanasia, the others being Luxembourg and the Netherlands. The Dutch also allow minors the right to die from the age of 12. Belgium will be the first to drop all age restrictions.

The contested legislation has already gone through the upper house of senate by a 3-1 majority and also passed parliament's justice committee.

The centre-left, liberals and Greens are expected to support the extension, while parts of the centre-right, allied with the Catholic church, which opposes euthanasia, will vote against.

"It is not about deciding whether a child is or is not to die," said Daniel Baquelaine, an MP who backs the new law and who is also a doctor. "Death is coming quickly. It is therefore necessary to allow the child to express what he thinks of the end of life, about how to die."

The extension of the right to die to minors is hedged with tight conditions. The bill states that the child must "be in a hopeless medical situation of constant and unbearable suffering that cannot be eased and which will cause death in the short-term".

Parental approval is needed, although it is not quite clear what happens if two parents of a child disagree. Psychiatric and medical advice is also required.

Although no age limits are set, the bill says that the child must display "a capacity of discernment and be conscious at the moment of the request".

It is not clear whether this rules out euthanasia for the very young. And the notion is contested by opponents.

A group of 160 paediatricians, opposed to giving children the right to die, said: "In practice, there is no objective method for determining whether a child is gifted with the ability of discernment and judgment. This is actually a largely subjective assessment and subject to influences."

The heated debate has split the medical, legal, and political professions, with the Catholic church staging prayer vigils in protest, and hospice and palliative care specialists stating themselves generally opposed.

Opinion polls show public majority support for the radical move. The paediatricians have called for a delay and further debate on the matter.

While supporters in the medical profession say such euthanasia is likely to be applied only in a few cases a year, the paediatricians claimed it was not necessary. They said they had never faced such demands; palliative care was now so sophisticated they could "fully control the physical pain, choking, or anxiety as they approach death".

They added that allowing euthanasia for children would create unbearable stress for carers and relatives.

"The care of seriously ill children is already complex enough. In such circumstances, it is even more difficult for medical staff and the family to be further faced with a particularly difficult ethical choice. The extension of the law to children will only increase their distress and stress."

Supporters said the bill would help bring ethical clarity and close loopholes – such as where doctors were quietly acceding to child euthanasia in excruciating circumstances but then facing prosecution for breaking the law.

There were 1,432 cases of euthanasia in 2012 in Belgium, according to official figures.

"The existence of a law is the best means of guarding against possible malpractice," said Le Soir, the main French language newspaper in Belgium.