The government’s failure to set a budget for the 2012 Olympic Games is a “major risk” to its chances of success, a report from the National Audit Office (NAO) has said.

The report from the Whitehall financial watchdog came just a week after the cross-party Culture, Media and Sport Committee said that the Treasury would have to find extra money, as London taxpayers’ contribution was at its limits.

The cost of the infrastructure and preparation of the Olympic park has increased by £900m on original estimates, to £3.3bn, prompting criticism that the project is threatening to run out of control.

The report says “a robust budget would allow the programme to move forward with greater confidence, with a better basis for financial control.”

Other areas for concern highlighted were the need for a clear and quick decision making process, and ensuring that the Government Olympic Executive is given sufficient expertise and personnel to effectively oversee the project.

The report is the first in a series from the NAO examining the delivery and financial arrangements put in place and charting overall progress.