Iran's parliament said on Wednesday President Mahmoud Ahmadinejad acted illegally by declaring himself caretaker oil minister and referred the case to the judiciary, increasing pressure on him to quit the post.

The legislature, which has repeatedly clashed with Ahmadinejad over key policy issues, voted to approve a report by its energy committee which found Ahmadinejad's move an "obvious violation of law", the semi-official Mehr news agency reported.

Last month Ahmadinejad sacked Oil Minister Massoud Mirkazemi as part of a plan to merge several ministries to cut their number to 17 from 21, alarming his rivals within the conservative ruling elite.

The president has the power to remove ministers and put caretakers in place for up to three months before having to consult parliament, and he says no one should be surprised by the reshuffle which has been on the cards for some time.

But taking personal control of the ministry that exploits Iran's vast oil resources was seen by some as a power grab to gain tighter control over the Islamic Republic's petro-dollars and a challenge to other arms of government.

"This illegal and hasty action will undermine the Islamic Republic of Iran's interests on the international level," the conservative dominated parliament said in its report.

"Mr Ahmadinejad as oil minister has issued some orders and will continue to issue orders which are obvious examples of illegal interference in governmental financial resources." - Reuters