The BBC has failed to show that its coverage of major sporting and music events provides the best possible value for money, a National Audit Office (NAO) report has found.
The NAO called for the Corporation to improve how it commissions coverage and evaluates success, with a more structured approach.
It said the BBC should set measurable targets for big events which can clearly demonstrate success and value - as the BBC "does not have transparency over the total budget for coverage of individual major events".
Total budgets were not brought together as several BBC divisions could be involved.
For example, BBC Sport's plan involved spending £13.7 million for TV, radio and interactive coverage of the Beijing Olympics. But this did not include extra budgets of £2.5 million for some talent, staff and online coverage, approved separately.
The NAO called for the BBC to consider a range of costed options and to conduct annual reviews assessing the quality and management of coverage.
The study focused on coverage of six events in 2008/09 at a cost of £35 million. They were the 2008 Beijing Olympics, the 2008 Uefa European Football Championships (Euro 2008), The Championships - Wimbledon, the Glastonbury Festival, the BBC Proms and Radio 1's Big Weekend.
The BBC spent £357 million in total during that period on the rights for and coverage of sporting and music events. The Corporation delivered all six events on time and largely within final budgets.
But Amyas Morse, head of the NAO, said: "The BBC has not done enough to demonstrate that its coverage provides the best possible value for money.
"When the BBC decides that it wants to cover a major sporting or music event, it should carefully explore a range of options, and set down clear objectives against which it can measure its achievements after the event. Without that, the BBC will not convince licence fee-payers that their money has been well spent."
