BRUSSELS — Belgian lawmakers clashed sharply on Wednesday over whether to grant terminally ill children the right to ask to die, a legal option already possessed by the country's adults.

The issue is pitting backers who see it as a question of mercy against opponents who claim the law is rushed, flawed and lacking medical rationale. Despite the spirited debate in the House of Representatives, the bill is widely expected to pass on Thursday.

"Our responsibility is to allow everybody to live, but also to die, in dignity," said Karine Lalieux, a Socialist member of the House of Representatives who favors extending Belgium's 2002 euthanasia law to minors under 18, with the added conditions that their parents approve and they understand what the decision means.

Sonja Becq, a Christian Democratic colleague, denounced the potential change, saying modern-day science is capable of relieving pain in very sick children until their illnesses runs their natural course.

"We cannot accept that euthanasia be presented as a 'happy ending'," she said. Belgium sets legal limits on who can legally acquire cigarettes and alcohol, she said — so why not for euthanasia?

The Senate, the other chamber of the federal legislature, adopted the legislation by a wide margin in December. If it passes in the House, all that would be needed for child euthanasia to become legal in Belgium is the signature of King Philippe, normally a formality.

The only other countries to have legalized euthanasia are two of Belgium's neighbors, the Netherlands and Luxembourg. In the Netherlands, children between 12 and 15 may be euthanized with parents' permission, while those who are 16 or 17 must notify their parents beforehand.

To guarantee that sick children understand what euthanasia signifies, Belgium's proposed legislation requires they demonstrate to a psychologist or psychiatrist that they possess the "capacity of discernment." Parents would also have to agree with their child's choice.

As with adults who seek euthanasia, the children would have to be in a state of unrelieved suffering, be afflicted with a terminal illness and be near death.

Foes of the law in the House said the proposal was full of holes.

"Can you tell me what a 'state of discernment' means?" asked Becq.

Does near death mean "three days, three weeks, six months?" asked Steven Vanackere, another Christian Democrat.

Daniel Bacquelaine, a member of the centrist Reform Movement, tried to assuage the opponent' fears.

"Where there is the smallest doubt about the discernment of the child, the question of euthanasia will not be posed," he said from the rostrum. The same goes when there is a glimmer of medical hope for the patient's survival, he said.

Though one survey has found 75 percent of Belgians in favor of extending the euthanasia law to children, there has been vocal opposition, with Andre Leonard, the Roman Catholic archbishop of Mechelen-Brussels, in the vanguard.

This week, an "open letter" carrying the names of 160 Belgian pediatricians was issued to argue against the new law, claiming there is no urgent need for it.

Other doctors have come out in favor, and say that only a handful of children, all teenagers, would be entitled to seek application of the law each year.