Shares in Barclays have fallen 13.5% after news that one of the largest Middle Eastern investors in the firm sold a stake worth about £3.5bn.
Abu Dhabi firm International Petroleum Investment Company (IPIC) sold 1.3 billion Barclays' shares, making a £1.5bn profit, according to Reuters.
In 2008, Barclays raised £7bn from Middle East investors to avoid using government funds to boost its finances.
But shareholders were unhappy, saying the move had diluted their stakes.
Barclays shares closed down 42.75p, or 13.5%, at 273.5 pence, the biggest loser among the FTSE 100 index of leading shares.
Of the £7bn raised last year by Barclays, £5.8bn came from Abu Dhabi and Qatar investors.
Late on Monday, IPIC - which is owned by the Abu Dhabi government - said it intended to sell Barclays' convertible notes equivalent to 1.3 billion shares.
On Tuesday traders said the shares were placed at about 267p each.
"The decision to dispose of some of its interests in Barclays reflects the focus of IPIC's long-term investment strategy on hydrocarbon-related opportunities," said IPIC managing director Khadem Al Qubaisi.
"The Emirate of Abu Dhabi intends to maintain a close commercial and strategic relationship with Barclays in the future," he said.
IPIC also said it would look into selling £1.25bn-worth of another type of investment in Barclays.