Clare Short, a former UK cabinet minister and an outspoken critic of the country's role in the invasion of Iraq, is appearing before Britain's Iraq inquiry.
Her testimony to the commission in London is of particular interest as Short has consistently been critical of Britain's decision to join the US in toppling Saddam Hussein, the then Iraqi president.
Since the invasion she has repeatedly branded Tony Blair, the British prime minister at the time, a "liar" who led the country to war.
Blair gave evidence last week in what was seen as the highlight of the inquiry.
The five-person panel is headed by John Chilcot, a former senior civil servant, and has been accused several times of being too soft on witnesses.
The Iraq inquiry was set up last year by Gordon Brown, who took over from Blair as the UK prime minister, to identify mistakes made, but it is not a trial.
Critical of Blair
In an article written in November 2009 and published on her website, Short wrote that "Blair lied to his cabinet, party and parliament in order to get us to war", but she has also been critical of the inquiry itself.
"The members of the inquiry team are all establishment insiders. Some are bright and likeable people but they have all spent a lifetime, not making waves. It is unlikely that they will start now," she wrote.
Short was among a number of Labour party members who opposed Blair's decision in 2003 to send 45,000 British troops to join the US-led invasion.
She resigned from her position two months after the invasion of Iraq and left the Labour party entirely in 2006.
She is now an independent member of parliament.
Short, who was the UK's international development secretary at the time of the invasion, is likely to be questioned on how her department was left out of the planning stages of the invasion.
A draft review of Iraq policy that was circulated among officials in 2001 was not forwarded to the international development department, which Short headed, prompting her to complain that it was being excluded.