The final legal arguments have been submitted to the European Court of Human Rights (ECHR) on behalf of a Belgian man contesting the euthanasia of his mother in 2012.
Godelieva de Troyer, 64, suffered depression but was physical healthy when she was euthanized by lethal injection.
Her son, Tom Mortier, did not find out about her decision until the day after she had been euthanized, when he was informed that she had "untreatable depression".
The ECHR agreed last year to hear the case after the Belgian authorities refused to review it.
ADF International, which is defending Mr Mortier, said that Belgium's euthanasia laws fail to protect the fundamental right to life. It says the case could set a precedent for euthanasia laws across Europe.
"International law has never established a so-called 'right to die'," said Robert Clarke, ADF International Deputy Director and lead counsel for Mr Mortier.
"On the contrary, it solidly affirms the right to life – particularly for the most vulnerable among us. One look at the tragic facts of this case exposes the lie that euthanasia is good for society.
"The sick, suffering, elderly, and vulnerable in our society deserve the utmost respect and care. As this case reaches its final stage, we hope that it will bring Tom some small measure of justice, and help protect others."
Belgium permits euthanasia where people are deemed to be in a "medically futile condition of constant and unbearable physical or mental suffering that cannot be alleviated, resulting from a serious and incurable disorder caused by illness or accident".
In 2014, the laws were extended to allow children to end their lives by euthanasia.
Paul Coleman, Executive Director of ADF International said: "The slippery slope is on full public display in Belgium, and we see the tragic consequences in this case.
"According to the most recent government report, more than six people per day are euthanized in this way, and that may yet be the tip of the iceberg.
"The figures expose the truth that, once these laws are passed, the impact of euthanasia cannot be controlled. Belgium has set itself on a trajectory that, at best, implicitly tells the most vulnerable that their lives are not worth living."