The BBC has seen a letter from Northern Ireland's attorney general, in which he questions whether a move to allow for abortion in cases of fatal foetal abnormalities is compatible with international human rights law.
The proposal is one of a number of amendments to the Justice Bill.
MLAs are due to vote on the issue at Stormont on Wednesday.
On Tuesday, the DUP asked the Northern Ireland health minister to set up a working group to examine it.
In the letter, John Larkin QC expresses concern to TUV MLA Jim Allister that the move may not be compatible with a United Nations Convention on the Rights of Persons with Disabilities.
Mr Larkin wrote that "providing for a criminal law exception for 'fatal foetal abnormality', as proposed by this amendment, provides unborn children diagnosed with such a disability with much less protection under the law of Northern Ireland than those without such a disability".
The attorney general pointed to cases in Spain and Austria, in which the UN committee which oversees the convention, has recommended that states should abolish any distinctions related to abortion in foetuses with disabilities.
Mr Larkin said: "those unborn children who are 'doomed to die' or are otherwise regarded by the proposers of this amendment as unworthy of life because of the nature of their disability" are as protected by the UN convention as any others.
He also said he thinks the 'conscience' provisions in Wednesday's amendments - proposed for NHS employees who have a religious or ethical objection to participating in abortions - are inadequate.
In January Mr Larkin lodged an appeal to a High Court ruling that found Northern Ireland's abortion legislation to be "incompatible" with human rights law.
A judge made the ruling in December.
A case had been taken by the Northern Ireland Human Rights Commission against the Department of Justice.
The judge said there should be exemptions in the law for women who were the victims of sexual crime and in cases of fatal foetal abnormality.
He said the current legal provisions in Northern Ireland breached the European Convention on Human Rights.
Unlike other parts of the UK, the 1967 Abortion Act does not apply to Northern Ireland, where abortions are illegal except where the life or mental health of the mother is in danger.