The newspaper said the Central Intelligence Agency's (CIA) findings that Iraqi President Saddam Hussein in 2003 was pursuing biological, chemical and nuclear programmes were based primarily on this old information.
However, despite the fact that the post-1998 evidence was largely circumstantial or "inferential," the panel said the CIA's belief that Saddam Hussein had weapons of mass destruction was justified, according to the report.
"It would have been very hard to conclude those programmes were not continuing, based on the reports being gathered in recent years about Iraqi purchases and other activities before the war," the Post quoted Richard Kerr, a former CIA deputy director who headed the four-person review panel as saying.
US intelligence agencies have been scrambling to cover up their failures in the period before the Iraq war.
Kerr said the intelligence reports given to the Bush administration prior to the invasion by the CIA, the Pentagon and State Department contained caveats and disagreements on data underlying some judgments, such as whether Iraq's nuclear programme was being reconstituted, according to the paper.
On another controversial Iraq intelligence issue, the panel's report indicated that although al-Qaida network and Saddam Hussein had a common enemy in the United States, "it was not at all clear there was any coordination or joint activities," said the Post, quoting an individual inside the CIA familiar with the report.
Iraq's alleged possession of weapons of mass destruction and links with al-Qaida were touted by US President George Bush among the key reasons for invading Iraq.