Portugal looks set Friday to approve a law legalising euthanasia for people in great suffering and with incurable diseases, joining the ranks of a handful of countries around the world.

The issue has divided this deeply Catholic country and witnessed strong opposition from conservative President Marcelo Rebelo de Sousa, a practising Catholic.

Under its provisions, people aged over 18 will be allowed to request assistance in dying if they are terminally ill and in unbearable suffering.

It will only cover those suffering "lasting" and "unbearable" pain - unless they are deemed not to be mentally fit to make such a decision. 

It will only be applicable for nationals and legal residents and not extend to foreigners coming into the country to seek assisted suicide.

The law was approved by parliament four times in the last three years but sent back for a constitutional review due to opposition from the president.

Once voted, the president will have eight days to promulgate it into law.

The ruling Socialists have vowed to have their way this time and if, necessary to hold two votes, on the same text.

"We have at last come to the end of a long battle," Socialist lawmaker Isabel Moreira told AFP.

Rebelo de Sousa had vetoed earlier bills due to "excessively undefined concepts" and later said the language used to describe terminal conditions continued to be contradictory and needed to be clarified.

The new version of the law now provides that euthanasia is only authorised in cases where "medically assisted suicide is impossible due to a physical disability of the patient". 

Rebelo de Sousa has asked lawmakers to specify who would "attest" to whether a patient was physically incapable of assisted suicide but lawmakers this time refused to modify the text.

"The adoption of this law has been relatively fast compared with other big countries," said Paulo Santos, a member of the "Right To Die With Dignity".

Critics regret that the issue has not been put to a referendum and hope opposition deputies will once again ask the Constitutional Court to look into the bill.

Euthanasia and assisted suicide are only allowed in a handful of countries including the Benelux nations and Portugal's neighbour Spain.