"God save us from money being given to a deputy asking him to vote for or against someone or for or against a bill," said Gholam-Ali Hadad Adel, who headed parliament from 2004 until he stepped down earlier this year.

Hadad Adel's remarks come two days after a stormy incident in parliament, in which media reported that a conservative deputy slapped the government's parliamentary representative, accusing him of vote buying.

According to the reports, MPs were being offered cheques for 50 million rials (5,215 dollars, 4,085 euros), ostensibly to benefit mosques in their districts. However, they were asked to sign an undertaking not to vote to censure Interior Minister Ali Kordan.

Government spokesman Gholamhossein Elham announced on Friday that President Mahmoud Ahmadinejad has ordered the opening of an investigation, semi-official Fars newsagency reported.

"The president has ordered a full inquiry ... If it appears that an illegal act has been committed, the government will act without fear or favour," Elham said.

Parliament will vote on Tuesday whether to impeach Kordan for "dishonesty" after he confessed to holding a fake Oxford University degree.

Kordan, who only took office in August, has been under pressure to resign after the prestigious British university denied awarding him any qualification through a representative, as he had claimed.

"The minister has claimed to have been deceived by the Oxford University's representative ... Whether he has been deceived or dishonest, such a person does not deserve to be interior minister," reads the censure motion.

Kordan later said he had approached Oxford University after MPs cast doubt on his degree, but "to my utter disbelief, the university did not confirm (the degree) when my representative went there."

He had shown the purported degree to MPs in a controversial vote of confidence on August 5, during which he was confirmed by a vote of 169-100.

Kordan replaced Mostafa Pour Mohammadi, who was at odds with President Mahmoud Ahmadinejad. The president has been supporting Kordan throughout the controversy.