The International Criminal Court Monday apologised for an embarrassing blunder during the trial of former Ivorian leader Laurent Gbagbo which led to the unmasking of several protected witnesses, and ordered a formal inquiry.

‘What happened on Friday afternoon... is of utmost and inexcusable gravity for which the chamber, but I would say the whole ICC, apologises,’ presiding judge Cuno Tarfusser said, offering a formal apology.

‘It is of such gravity that the chamber has ordered an ... internal investigation in order to find out how this could have happened,’ a visibly upset Tarfusser said.

He was speaking as the second full week of Gbagbo's crimes against humanity trial began in the international tribunal based in The Hague.

On Friday the prosecutor mentioned the names of several witnesses whose identities have been ordered to be kept secret, thinking the microphones were off.

But the incident was relayed live to the public gallery, and recordings have since spread widely on social media, including YouTube.

The judge added he did not know whether Friday's incident happened because of ‘recklessness, superficiality (or) stupidity,’ adding he did not want to ‘speculate about something else.’

‘We are awaiting a detailed report,’ Tarfusser added.

Protecting witnesses is one of the key promises of the ICC as it seeks to bring to justice those responsible for the world's worst crimes.

The tribunal, set up in 2002, goes to enormous lengths to shield the identities of sensitive witnesses from the public, pixellating their faces and disguising their voices. In some cases, witnesses who fear reprisals are moved to a new country and given a new identity.

The trial of Gbagbo, 70, and his firebrand youth militia leader Charles Ble Goude, 44, opened on January 28, and is set to last three to four years.

The two men have denied four charges arising out of their alleged roles in post-election violence that ravaged the west African country in 2010-11.

- Criminal offence -

But on Friday prosecutor Eric MacDonald inadvertently gave the names of at least three witnesses, in a blunder which made headline news in the Ivory Coast.

It provoked outrage from Gbagbo's supporters and those of his bitter rival, Ivorian President Alassane Ouattara, over the weekend.

Tarfusser urged everyone to ‘calm down as we don't want to ignite anything. We just want to do our jobs as best as possible.’

He also warned that any attempt to interfere with or unmask witnesses was a criminal offence, reminding all involved ‘to do nothing to reveal or continue to publish... the identities of protected witnesses.’

On Wednesday, witness P547 also accidentally gave his name as he recounted how forces loyal to Gbagbo allegedly fired on unarmed protesters.

Some 3,000 people were killed after the disputed Ivory Coast presidential vote in late 2010, when Gbagbo refused to cede victory to Ouattara, who was backed by international powers after snatching a narrow victory.

After the apology, Gbagbo's trial resumed with defence lawyers continuing a cross examination of the prosecution's first witness, known only as ‘P547’.