The head of the commission that investigated the Sept 11 terrorist attacks had closer ties to the White House than he admitted and tried to limit the Bush administration's responsibility for the incident, a book claims.

Philip Zelikow, the 9/11 Commission's executive director, allegedly attempted to intimidate staff to avoid findings that would be damaging to President George W Bush, who was running for re-election, and Condoleezza Rice, his then National Security Adviser.

The claims are made by Philip Shenon in The Commission: The Uncensored History of the 9/11 Investigation, published today.

Although it was known that Mr Zelikow was a friend of Miss Rice and that they had written a book together in 1995, Mr Zelikow had pledged not to talk to senior White House figures during the investigation.

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However, Mr Zelikow told Shenon, a New York Times reporter, that he spoke to Miss Rice several times during the 20-month inquiry and also exchanged frequent calls with the White House, including four to Karl Rove, Mr Bush's then chief political adviser.

The book says Mr Zelikow, a former academic, once tried to push through wording in a draft report that suggested a greater tie between Osama bin Laden and Iraq, in line with White House claims but not with the views of the commission's staff.

The book seeks to raise new questions about the independence of the bipartisan commission, which was created in 2002 to investigate government mistakes that led to the Sept 11 attacks.

In July 2004, it published a 567-page report that blamed neither Mr Bush nor Bill Clinton, his predecessor, although it noted that neither had made anti-terrorism a priority.

According to the book, when Bob Kerrey, a Democrat member of the commission, learned the extent of Mr Zelikow's ties to the administration, he confronted Tom Kean, its Republican chairman.

Mr Kerrey reportedly threatened to resign unless Mr Zelikow was sacked, but was persuaded to stay.