The French trader that incurred massive losses for Societe Generale was in profit by 1.4bn euros at the start of the year, the BBC has learned.
Mr Kerviel will not be investigated for fraud
But as European stock markets started to fall the bank lost 500m euros for every 1% drop in share prices.
This means that his bet was big enough to bankrupt France's second largest bank on the basis of any sustained fall in stock markets.
Jerome Kerviel's actions cost Societe Generale 4.9bn-euros ($7bn; £3.7bn)
The BBC's business editor Robert Peston has also learned that Societe Generale's back office, or administrative operation, queried Mr Kerviel's transactions as long ago as March or April last year.
"Bankers have confirmed that at the end of last year, Jerome Kerviel had generated a colossal hidden profit for the bank of 1.4bn euros," Mr Peston said.
"Among the great mysteries of the Kerviel affair is how the French bank could have failed to notice a profit of that size."
Mr Kerviel said he "did not believe" the bank's senior management would have been unaware of the risky bets he was taking, according to testimony published in the French daily Le Monde.
"It's impossible to generate such large profits with small positions, which leads me to say that when I'm in the black, my superiors closed their eyes about the methods and volumes committed," he was reported as saying.
Breach of trust
Mr Kerviel, 31, is being investigated for breach of trust, falsifying documents and breaching computer security - but not the more serious charge of fraud.
Societe Generale says Mr Kerviel had an unauthorised position, or a bet, worth about 50bn euros on the future direction of European shares.
To avoid that potentially catastrophic loss, the bank had to unwind Mr Kerviel's trades, but that still cost it 4.9bn euros.
Societe Generale said Mr Kerviel's experience in administrating trades enabled him to bypass strict risk controls.
It said he invented deals that, on paper, balanced out his bets.