The White House has admitted in court that it has lost three months of email backups from the initial days of the Iraq war, raising questions about the possible deletion of politically sensitive records.

The disclosure came in a lawsuit filed by the National Security Archive, a non-profit group that specialises in uncovering classified documents.

The archive was told it could not receive emails relating to Iraq, despite a 30-year-old law requiring the preservation of presidential records, because a system upgrade had deleted up to 5m emails.

George Bush's administration faced a deadline of this week to outline the contents of 438 backup tapes that were believed to contain emails deleted in 2003 between March—when the US invaded Iraq—and September.

In a court filing yesterday, however, White House chief information officer Theresa Payne said "the earliest date" with email on any of the tapes was May 23 2003: the date the UN gave formal approval to the US occupation of Iraq.

"What is most shocking is that if anyone at the White House was deleting their emails during the invasion of Iraq, those e-mails are not on any backup tapes," Tom Blanton, director of the archive, said.

Payton also told the US courts that verifying the contents of the tapes between late May and September of 2003 would be overly costly and an administrative burden.

Congress has begun an investigation of its own into a White House recordkeeping under George Bush, whose aides eliminated an automatic email archive programme used by predecessor Bill Clinton.

The loss of White House emails also proved an obstacle to Patrick Fitzgerald, the special prosecutor investigating the leaked identity of CIA officer Valerie Plame.

Fitzgerald said in 2006 that he believed some potentially relevant emails sent by aides in vice president Dick Cheney's office were lost in the Bush administration's email system.