Mr de Mistura said the clause had to be reinstated into the law by 15 October
The UN special representative for Iraq has criticised its MPs for dropping an election law clause which was designed to protect minority rights.
Staffan de Mistura said he was "surprised and disappointed" the clause had been omitted and called for it to be reinstated as soon as possible.
Article 50 would have guaranteed a certain number of seats on provincial councils for Iraq's many minorities.
On Sunday, Christians protested against the legislation in Baghdad and Mosul.
Their leaders said the new law, passed to international acclaim last month, was unfair and an indirect attempt to force them to leave.
Iraq's ethnic and religious minorities have been targeted mainly by Sunni extremist groups since the US-led invasion in 2003.
'Fundamental for democracy'
In a statement on Thursday, Mr de Mistura said he had been concerned by the removal of the quota system for minorities from the election law.
"I was surprised and disappointed that Article 50 was not included in the provincial elections law," he said.
"Article 50 has the backing of minority groups, political blocs and UNAMI [United Nations Assistance Mission for Iraq] and should now be reinstated into the legislation as soon as possible so minorities can participate in the upcoming elections to be held sometime before 31 January 2009."
Mr de Mistura emphasised that protecting the human rights of minorities was fundamental to a democratic Iraq.
"Article 50 is a strong indication Iraq is a nation ready to protect the political rights of minorities as founded in the constitution," he added.
The provincial election law was passed only after months of debate by MPs
The UN envoy promised to continue consultations with Iraqi political leaders to ensure that the clause was reinserted into the election law by the Council of Representatives before 15 October, when the electoral commission opens nominations for candidates.
After Sunday's protest by Christian groups, Prime Minister Nouri Maliki urged the election commission to ensure that minority rights were protected.
"We are committed to guarantee a fair representation of all Iraqi components and defend their rights," he said in a statement.
Correspondents say the provincial elections are part of a US-backed plan to reconcile rival groups, particularly Sunni Arabs, who boycotted the last round in 2005.
The law allowing them to be held before the end of January in 14 of Iraq's 18 provinces was passed only after months of debate over how it would be applied to the oil-rich city of Kirkuk.