The organisation, which is an arm of the Qatar Investment Authority, bought the shares a year ago to help the bank avoid the need for a government bailout.
But it has now said it wants to sell 379 million of its shares in Barclays.
The news saw shares in Barclays fall five per cent in London trading.
Qatar Holding chief executive Ahmad Al-Sayed said the company would continue to hold around seven per cent of shares in the bank, meaning that it will remain Barclays's largest shareholder.
Barclays chief executive John Varley said of the sale: "The effect will further broaden the base of our share register.
"Qatar Holding is our largest shareholder and a key partner of the Barclays Group."
Last month, Barclays acquired the Portuguese credit card business of Citibank, giving it around 400,000 extra customers in Europe.
By Tony Aynsley