The hack also involved the planting of damning false stories about Qatar's emir.
The Emirati ambassador to the United States, Yousef al-Otaiba, was also quick to deny the report.
The report "unequivocally proves that this hacking crime took place", it added.
The false remarks by Emir Al Thani were quickly picked up and circulated by other Gulf news outlets, even after Qatar issued a statement clarifying the story was fabricated.
The Post said officials became aware of the UAE's role in the hack after newly analyzed information gathered by members of the US intelligence community revealed senior members of UAE's government discussed the plans and how to implement the attack.
The statement cited the government's communications director, Sheikh Saif bin Ahmad al-Thani, as saying it was "especially unfortunate that this shameful act of cyber terrorism is being attributed to a fellow member of the Gulf Cooperation Council".
In late June, Kuwait, as a mediator in the crisis, handed over to Qatar the ultimatum of the four Arab states with 13 demands, including the requests to severe Doha's relations with Tehran, close Turkey's military base in Qatar and shut down the Al Jazeera TV channel, as well as to end support for the Muslim Brotherhood, a terrorist organization banned in Russian Federation.
USA investigators in June told CNN that they believed Russian hackers breached the Qatari sites and planted false information and quotes. It added that the "crime" against its state news agency by a Gulf nation was a violation of worldwide law.
It is understood the FBI has been assisting Qatar with an investigation into the hack for several weeks.
The Washington Post reports that new intelligence reveals the UAE government met to discuss the hacking plot and its execution on 23 May.
Qatar - which shares its only land border with Saudi Arabia - has rejected accusations it supports terrorism, calling them "unjustified" and "baseless". "They [the UAE] claim that basically their demands from Qatar are legitimate and they insist that they should be fulfilled nearly as a diktat instead of sitting at a table and negotiating some legitimate differences that they might have with Doha, and somehow, unfortunately, Saudi Arabia has followed suit", said Jahshan. Qatar says this is part of it's open-door policy, and the aim is to encourage dialogue, not to give backing to extremists.
U.S. spies accused the UAE of infiltrating the sites to plant fake news and provoke one of the worst diplomatic rifts in recent Middle East history.