The shares were sold as part of the liquidation of Kepit, an investment trust specialising in European stocks. Warburg chose the market turmoil that surrounded Wednesday's surprise increase in interest rates to execute the trade. Later in the day prices rose back to more advantageous levels.
Merrill Lynch, which is advising Kepit, accused Warburg of executing the trade at poor prices. The row was later defused after Warburg offered to use closing prices for the stocks.
One source said: "It's no big deal. This sort of thing happens the whole time." But another said it was indicative of the way market-makers manipulated prices to their own advantage.
"This was a programme trade," a source close to the transaction said. "The timing had been pre-set and it was unfortunate that it occurred when prices were weak as a result of the interest rate decision."