The US fund has acquired shares in the British bank and is attempting to stymie the deal since it perceives that Barclays' stock is undervalued, but that by scrapping its buyout bid its share price will increase.
However, the plans appear to have been rejected by Barclays, which told the Wall Street Journal: "The views expressed by Atticus Capital LP are not representative of the feedback we have received from shareholders who remain supportive of our strategy.
"If other shareholders feel differently, we encourage them to engage in a dialogue with us. We believe this transaction will create significant incremental value for our shareholders and meets our rigorous financial criteria."
The size of the stake acquired by Atticus is not known, but is estimated to be around one per cent.
Barclays has tendered a bid of $84.6 billion for ABN Amro, meanwhile a consortium headed by Royal Bank of Scotland has launched a counter-bid of $94 billion.