MPs believe Mr Blair, who was in office between 1997 and 2007, should be prosecuted for breaching his constitutional duties and taking the country into a conflict that resulted in the deaths of 179 British troops.
Not used since 1806, when Tory minister Lord Melville was charged for misappropriating official funds, the law is seen in Westminster as an alternative form of punishment if, as believed, Mr Blair will escape serious criticism in the Chilcot Inquiry report.
Triggering the process simply requires an MP to propose a motion, and support evidence as part of a document called the Article of Impeachment.
If the impeachment attempt is approved by MPs, the defendant is delivered to Black Rod ahead of a trial.
A simple majority is required to convict, at which point a sentence can be passed, which could, in theory, involve Mr Blair being sent to prison.
Last year, current Labour leader Jeremy Corbyn said the former prime minister could be made to stand trial for war crimes, saying that he thought the Iraq War was an illegal one and that Mr Blair “has to explain that”.
He added: “We went into a war that was catastrophic, that was illegal, that cost us a lot of money, that lost a lot of lives.
“The consequences are still played out with migrant deaths in the Mediterranean, refugees all over the region.”
It is believed the 2.6 million-word report, due to be published in July, will not make “any judgements on the legality [of the Iraq War]or anything like that, that is not the purpose”.