The inquiry panel, which started work in 2009, had to analyze some 150,000 government documents covering a nine-year period, Chilcot told parliamentarians while giving evidence on delays in the preparation of the report.
Chilcot added that publication of the report was also stalled because of the scope and complexity of the inquiry.
The current inquiry seeks to uncover the operations of the United Kingdom in the Iraq War, including British policies and military action.
The inquiry report, known as the Chilcot report, was to be published by the end of 2014, but according to UK Cabinet Office Minister Lord Wallace, the government had decided it would be inappropriate to publish the report prior to the UK general election in May.
The invasion of Iraq in 2003 aimed to eliminate weapons of mass destruction believed to have been held by the late Iraqi leader Saddam Hussein. The war claimed the lives of more than 100,000 civilians between 2003 and 2011, according to the Iraq Body Count database.
