Tony Blair’s government believed the UK would spend no more on the 2003 invasion of Iraq than it did on the 1991 Gulf war, according to documents released under freedom of information rules.

In an insight into how British officials failed to predict both the length of the UK engagement in Iraq and the financial drain, Treasury documents from September 2002 show it greatly underestimated the costs, believing British troops would need to remain “fully engaged” in Iraq for just six months.