The Conservative leader tried to taunt Gordon Brown at Prime Minister's Questions, accusing him of getting the age of Renaissance artist Titian wrong.
Within seconds, they realised they had gaffed as art historians insisted the true age of the painter, born in the 15th century, has never been known.
But rather than admit a mistake, panicking staffers at Conservative Central Office changed Wikipedia to make sure the online encyclopaedia agreed with their leader.
A Tory spokesman later blamed an "over-enthusiastic member of staff".
But a source lamely tried to claim they wanted to "correct" the page, rather than cover up Cameron's gaffe.
The source said: "We don't think it is our business to change entries in Wikipedia.
"In this case, an over-eager member of staff put right an entry which was clearly wrong."
But Labour MP Chris Ruane said: "We are used to Tories issuing tissues of lies. This time it's Titians of lies."
The gaffe was discovered because the computer registration number which was recorded alongside the change made on Wikipedia was traced back to Conservative Central Office within minutes.
The year the painter died was altered to back up Cameron's claim at 12.34pm - four minutes after Prime Minister's Questions.
The bizarre Westminster row began after a speech by Brown at the World Economic Forum in Davos last month.
He jokingly compared world leaders' struggle with the financial crisis to the Italian painter.
He said: "I'm reminded of the story of Titian, who's the great painter who reached the age of 90, finished the last of his nearly 100 brilliant paintings, and he said at the end of it, 'I'm finally beginning to learn how to paint,' and that is where we are."
During Prime Minister's questions, Cameron said: "You told us the other day you were like Titian aged 90."
As Tories MPs laughed, he said: "The fact is Titian died at 86."
According to Encyclopaedia Brittanica, Titian was born sometime between 1488 to 1490 and died in 1576. Titian himself claimed he was born in 1477.
Yesterday, the Wikipedia entries pages were changed a number of times.
Labour MP Peter Kilfoyle later poked fun at Cameron's embarrassment, asking the deputy Speaker: "I wonder whether you could tell the House whether you have had representations from the leader of the opposition so that he might correct the comments that he made about Titian.
"Or is it enough in this modern age for the leader of the opposition's staff simply to alter Wikipedia?"
Titian expert Caroline Campbell, of the Courtauld Institute of Art, said: "It is certainly not a fact Titian was 86 when he died. The artist himself actually claimed he was over 100 years old near the end of his life.
"The problem is we know when he died - in 1576 - but not when he was born because records of his birth have never been discovered.
"Saying he was 86 is a guess. We simply don't know for sure."