The extension was requested by the commission, which said it needed the 27 May deadline pushed back by 60 days to 26 July in order to complete interviews and review documents related to the attacks.
"We are pleased to support their request for an extension and we urge Congress to act quickly to extend that timetable," White House spokesman Scott McClellan said on Wednesday.
Some lawmakers wanted to delay release of the commission's potentially damaging report until after the November presidential election.
The White House had until now resisted calls from the commission to approve any extension, arguing it should finish its work on schedule.
But pressure has been mounting on the White House to back down - from relatives of 11 September victims and the commission itself.
Members of the panel have complained that the administration has been slow to provide the documents they need.
The White House and the commission are still trying to settle another dispute - over access to notes taken by panel members from highly classified presidential briefing papers.
A November agreement allowed four members of the commission to review documents known as the President's Daily Briefs, including one from August 2001 that warned of the possibility of an al-Qaida plot to hijack aeroplanes.
But the White House has balked at allowing the notes to be shared more broadly among commissioners and investigators.
Republican Senator John McCain of Arizona and Democratic Senator Joe Lieberman of Connecticut have introduced legislation that would delay the commission's deadline until January 2005, well after the election.