Tony Blair was urged by a senior British officer two days before the invasion of Iraq to delay the military offensive because preparations for dealing with the aftermath were not ready, it was revealed at the Chilcot inquiry today.

Concern about the legality of the invasion and its cost, combined with a complete lack of focus in Whitehall, prevented the Blair government from preparing adequately for the consequences in Iraq, the inquiry heard.

Major General Tim Cross, the only UK military official appointed to help plan the invasion aftermath, delivered a severe indictment of Whitehall's failures. "There were serious concerns about legality and over money," he said.

The Blair government was reluctant to sign up to the US body responsible for postwar planning for fear of being "liable under international law", he added.

Cross, who was attached to the Office for Reconstruction and Humanitarian Assistance, set up by the US to manage the aftermath, described the preparations as "woefully thin". In a written statement, he said he had briefed Tony Blair at No 10 on 18 March 2003, two days before the invasion.

"We talked for about 30 minutes or so. I was as honest about the positions as I could be, essentially briefing that I did not believe postwar planning was anywhere near ready," said Cross. "I told him that there was no clarity on what was going to be needed after the military phase of the operation, nor who would provide it.

"Although I was confident that we would secure a military victory, I offered my view that we should not begin that campaign until we had a much more coherent postwar plan."

Describing his meeting with Blair in oral evidence, Cross said: "He was engaged. I gave him the background of what we had been doing. We had a very sensible conversation. At the end of it I remember saying, in so many words, I had no doubt we will win in the military. I do not believe we are ready for postwar Iraq.

"He nodded and didn't say anything particular. I didn't expect him to look me in the eye and say, 'This is terrible, we are going to pull the whole thing off.' I was just one of a number of people briefing him."

Cross added: "I hesitate to say I used the word 'disaster'. I may well have used the word 'chaotic'."

Cross confirmed what the inquiry was told by Lord Boyce, then chief of the defence staff – that the government also delayed military preparations because it did not want the public to know it was planning for war. Cross described how in the weeks before the invasion there was "no minister of cabinet rank driving this from day to day". He continued: "I felt very isolated. There was no sense diaries were being cleared even for war fighting."

Cross told the inquiry in his written statement: "Not everyone believed that there would actually be a war; if there was to be one, then there was certainly no consensus that we (the UK) should be involved; there was no coherent UK … view of what postwar Iraq should look like; there was a serious reluctance to take on the US over their views … there was therefore some seriously wishful and woolly and 'unjoined' up thinking going on."

Cross referred to concern about the costs of rebuilding Iraq in the Treasury and in the international development department, under Clare Short, where officials were also worried about the legality of the invasion. Short resigned shortly after the invasion.