There should be tighter controls on the management of the BBC's website - in part to prevent it stifling commercial rivals, the BBC Trust has said.
After reviewing bbc.co.uk, the trust said it was "an excellent service" but that its content had to be different from that on rival websites.
And it criticised the site's management for a £3.5m overspend last year.
The report is the first of a number of reviews of the BBC's services as part of its new charter and agreement.
The trust, an independent body which works on behalf of licence payers, has taken responsibility for such reviews from the culture secretary to establish greater independence for the BBC from the government.
In its report, the trust said the BBC website delivered great public value to the "vast numbers who use it [and] love it".
It quoted figures from online statistic company Nielsen suggesting the site had 16.5 million UK users each month by December 2007.
The site continued to grow despite competition, the report added.
Representatives of competitors, including ITN and the Newspaper Society, were consulted as part of the review.
Presenting the reports findings, review leader Dame Patricia Hodgson said: "This is a very reassuring set of findings about the quality of the service, the value that it gives to audiences and what they think about it."
She added that there would be three new safeguards to protect competitors:
- Each part of the website to have its own aims and budget
- Each part of the website to be tested against seven criteria to find out if it is distinctive enough
- Any major new development in bbc.co.uk that could impact on other sites - including local news sites - to be reviewed with a "public value test"
The trust has told BBC management there must also be tighter controls on the financial side.
Some reports suggested the BBC's website overspent by £36 million in 2007/2008, but Dame Patricia said there had been a "small" overspend of £3.5 million.
Her figure was based on the money spent by bbc.co.uk, £85.1 million, compared to its absolute maximum budget of £81.6 million.
The original budget was £74.2 million - but the BBC had a leeway of 10% either way on this figure.
In its review, the trust also uncovered the "£24.9 million misallocation of general overheads and costs from other budgets".
Dame Patricia said the money was not wrongly spent but was simply allocated to other departments during restructuring.
"That may be the explanation but clearly it is not acceptable," she said.
"There must be stronger financial, strategic and editorial controls."
Neither the £24.9 million misallocation or the £7.4 million difference between the baseline budget and the maximum budget were actual overspends, the trust added.
The trust is also calling for an improved internal search engine, more links to non-BBC websites and more interactivity.
It said any plans for new investment in bbc.co.uk would not be approved until management had made satisfactory plans to further tighten controls.