Iran's conservative Guardian Council has revised its ban on a further 100 people wanting to stand for election.
Ayatollah Jannati said a review would be completed by 30 January
About 350 disqualifications have been lifted since Iran's spiritual leader told officials to find a solution.
But thousands of candidates, said to be mostly reformists, are still banned from February's parliamentary poll.
The qualification row has prompted a political crisis, with high-ranking officials threatening to resign if the ban is not lifted.
The head of the Guardian Council, Ayatollah Ahmad Jannati, said the assessment of candidates was the group's "most important issue at present".
"We are working day and night to investigate meticulously," he said during an address for Friday prayers which was broadcast on state radio.
'Change the law'
But he also used his sermon to criticise the process of open elections in Iran which led to the council having to scrutinise thousands of would-be candidates who put their names forward.
"The law must be changed one day. The law must be amended one day," Ayatollah Jannati said.
"It should not allow everyone to compete as a candidate by bringing a birth certificate, four passport-size photographs and some qualifications. By doing so, we suddenly found out that 30,000 candidates had registered their names."
He said mistakes were almost inevitable and that the council would do what it could to rectify them by a deadline of 30 January.
The original ban on 3,500 candidates has now been lifted in about 10% of cases, following a plea for a review by the Islamic Republic's spiritual leader, Ayatollah Ali Khamenei.
The disqualifications from February's parliamentary election - which included some people who were already MPs - provoked outrage among reformists.
Dozens refused to leave the parliament building and senior government officials threatened to resign if the election was not open.
Iran's reform-minded elected President, Mohammad Khatami, has appealed for calm and now says he believes the polls will be fair and competitive.
In his sermon, Ayatollah Jannati praised the guidance of Ayatollah Khamenei.
"He asked us to be generous in our assessment and we have borne this in mind and continue to practise it at our meetings," he said.
But he also used his broadcast address to lash out at comments from overseas, including views expressed by the US and European Union.
"I am ashamed to see that foreign powers have become so presumptuous to interfere in our affairs," he said.
He suggested they should be "ashamed of their behaviour".