His explosive blunder came as he stood in for David Cameron at Prime Minister's Questions in the Commons for the first time.
During clashes with Labour's deputy leader Jack Straw, Mr Clegg declared: "Perhaps one day you could account for your role in the most disastrous decision of all, which is the illegal invasion of Iraq."
His outburst left Tory ministers - who all voted for the invasion - looking stunned.
Within hours, Number 10 rushed out a statement stressing that Mr Clegg was merely expressing a "personal" view.
But officials stoked confusion by adding that the coalition had not decided if the Iraq War was legal or not.
They said the Iraq Inquiry was looking at the legality of the war. But the hearing has made clear it was "not a court of law".
Mr Clegg's comments were criticised last night by ex-Army chief General Sir Mike Jackson.
He said: "Nick Clegg was less than well advised to give his opinion in these circumstances." And senior Tory MP Patrick Mercer warned: "This is very worrying for those who took part in the conflict.
"It gives a clear go-ahead for those who claim some, or all, of our troops' actions in Iraq were illegal."
And Labour MP Michael Dugher added: "Our armed forces do a great job and when they go to war for our country they deserve their government's full support."
In a second gaffe, Mr Clegg told MPs the Yarl's Wood immigration detention centre at Clapham, Bedfordshire, was being closed "for good".
But within hours, the Home Office said only one section of the centre - where children are held - was being shut.
It stressed Yarl's Wood will "continue to function as an immigration removal facility for adults".
Meanwhile, Lib Dem support has plummeted to just 13 per cent, the lowest since Menzies Campbell was toppled as leader.
A YouGov poll for The Sun reveals the Lib Dems, who have 57 MPs, would be left with just 15 if a General Election were held now.
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