Qatar said in late May that hackers had posted fake remarks by the emir, an explanation rejected by Gulf states.
UAE Ambassador Yousef al-Otaiba denied the report in a statement, saying it was "false", the Post said.
"The Washington Post story is not true, purely not true", he said responding to a question after a speech at Chatham House in London. He said that the story "will die" in the next few days.
In an interview, Mohammed Cherkaoui, professor on conflict resolution at George Mason University, said the new Washington Post "bombshell" shows that the Gulf crisis "is moving from one escalation to another".
Still, the four Arab powers have said the memorandum fell short of allaying their concerns that their sanctions would remain in place until Doha meets their demands and that they would keep a close eye on Qatar's efforts to fight terrorism funding. The bogus remarks were reported by Saudi-supporting media.
The Federal Bureau of Investigation was previously known to be working with Qatar to probe the hacking.
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The Washington Post gave no further details of how American intelligence had reached its conclusion, but it has previously been alleged that some of the boycotting states could be behind a hack of the official Qatar news agency.
The GCC is a six-member bloc that includes Bahrain, Kuwait, Oman, Qatar, Saudi Arabia and the UAE.
"You can not be both our friend and a friend of al-Qaeda", he said, repeating allegations - denied by Qatar - that the country funds extremists.
"What is true is Qatar's behaviour. Inciting violence, encouraging radicalization, and undermining the stability of its neighbors", the statement said.
Qatar has acknowledged providing assistance to Islamist groups designated as terrorist organisations by some of its neighbours, notably the Muslim Brotherhood. His visit had yielded little except for a bilateral agreement between the United States and Qatar to fight "terrorism".
Mr Gargash said Qatar's neighbours were prepared to continue the boycott for months if it did not comply with the list of demands it was handed last month and agreed to global monitoring.
But, he added, the four states would not escalate the boycott by asking companies to choose between doing business with them or with Qatar.