UAE Ambassador Yousef al-Otaiba denied the report in a statement, saying it was "false", the Post said.
Qatar has maintained its emir never made the remarks and that the quotes were planted by hackers.
Speaking at the Chatham House forum in London on Monday, the UAE foreign minister, Anwar Gargash, twice denied the veracity of the claims.
"The information published in the Washington Post on 16 July 2017, which revealed the involvement of the United Arab Emirates (UAE) and senior Emirati officials in the hacking of Qatar News Agency, unequivocally proves that this hacking crime took place", Qatar's government said in a statement on Monday.
Two weeks later, the four countries cut all links with Qatar over its alleged support for terrorism and relations with Iran.
Qatari officials said the agency had been hacked by an "unknown entity" and that the story had "no basis whatsoever".
Swiss news network The Local said a fake news story quoting Federation Internationale de Football Association president Gianni Infantino had been posted on a copycat website on Saturday.
But according to USA intelligence agencies, in a report leaked to the Washington Post, the hack was planned by UAE government officials, although it is unclear whether UAE government agencies carried out the cyber attacks directly or contracted a third-party to do it for them to keep it at arm's length.
Later that day, the official Qatar News Agency quoted Emir Sheikh Tamim bin Hamad al-Thani as criticising USA "hostility" towards Iran, describing it as an "Islamic power that can not be ignored", and calling Hamas the "legitimate representative of the Palestinian people". Qatar has previously asked U.S. and British officials to investigate the source of the hack.
The GCC is a six-member bloc that includes Bahrain, Kuwait, Oman, Qatar, Saudi Arabia and the UAE.
"What we really do want is we either reach an agreement and Qatar's behaviour changes, or Qatar makes its own bed and they can move on and we can move with a new relationship".
"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. "Inciting violence, encouraging radicalisation, and undermining the stability of its neighbours".
"You can not be both our friend and a friend of al-Qaeda".
"I understand the concern of our allies", he added. A few days later the four states launched their blockade of Qatar.