A protester interrupted Tony Blair's testimony at the Leveson Inquiry on Monday, shouting: "The man is a war criminal!"
The intruder, a white male in his 40s, was forcibly removed from the room after entering through the non-public part of the Royal Court of Justice.
Before being wrestled out of the room, the protester shouted: "This man should be arrested for war crimes... JPMorgan paid him off for the Iraq War... three months after he invaded Iraq he held up the Iraq bank for £20bn. He was then paid $6bn a year by JPMorgan and still is... the man is a war criminal."
After the incident, a man identified himself to reporters as David Lawley Wakelin from the Alternative Iraq Enquiry. He spoke as security guards escorted him through the Royal Courts of Justice.
It is understood the protester, who burst into the hearing directly behind Lord Justice Leveson, managed to get past security-coded doors to access the judges' corridor leading to courtroom 73. He was subsequently arrested and released without charge.
Following the incident, a different protester threw an egg at the former PM's car as he left the inquiry, according to reports.
Lord Justice Leveson has ordered an immediate investigation into how the man gained access.
He said: "I would like to find out how this gentleman managed to access the court through what's supposed to be a secure corridor and I'll have an investigation undertaken about that immediately."
Blair remained calm throughout the disturbance.
After the removal of the protester, Mr Blair denied his allegations.
He told the hearing: "What he said about Iraq and JP Morgan is completely untrue.
"I've never had a discussion with them about that."
A spokesman for Her Majesty's Courts and Tribunals Service said the Service took security very seriously at all of their venues.
"An investigation has been ordered into an incident at the Leveson Inquiry, Royal Courts of Justice, today.
"It would be inappropriate to pre-empt the findings of this investigation," he added.
When the hearing reconvened for the afternoon, Lord Justice Leveson told the court the Inquiry and HMCTS took the incident "extremely seriously" and apologised for the breach.
He said: "Considerable effort has been put into ensuring all witnesses can give their evidence in a safe and secure environment and I very much regret what has happened.
"An investigation is being undertaken and I will be giving consideration to the steps that can be taken and should be
taken against this particular intruder.
"Efforts will be redoubled to ensure that incidents of this nature don't recur.
"I repeat my apologies to Mr Blair and indeed to everyone else who was involved in or following our inquiry."
Earlier, Blair said he felt the power of the British media was unhealthy while he was PM - but chose not to take it on as it would have prevented him from pursuing any other policies while in power.
On arriving to give evidence at the inquiry, the former PM had been greeted by around two dozen or so protesters as he arrived at the courts this morning.
They waved banners reading "Troops home", "Bliar" and "Afghanistan out".