It is not every week that the BBC is told that its integrity has been compromised, as it was told on Monday by the BBC Trust, but what is surprising is how long the public broadcaster took to reach the same conclusion. It has been quietly taking money from advertisers - £5.9 million over the past five years - by way of “programme sponsorship”, but the licence-fee-funded broadcaster has not felt the need to admit a problem.

When Rupert Howell, ITV's new commercial director, sat down to watch BBC Sports Personality of the Year in December, he was amazed to find the event sponsored by Robinsons, the squash maker, whose brand was shown in a BBC One programme watched by 8.6 million people. He later joked: “Which is the most commercial broadcaster in Britain? The BBC.”

ITV estimated that the BBC had received £200,000 in a two-year deal, which defrayed the event's costs. Taking advantage of rules that allow it to cover sponsored events, such as the FA Cup, the public broadcaster had raised cash over several years to part-fund programmes that could be redefined as events. Because Sports Personality of the Year was transmitted live from the NEC, with Joe Calzaghe winning in front of an audience of 8,000, it could be classified as an event.

Last year the BBC took £955,000 from sponsors, not a lot beside the licence fee's £3.4 billion. It was a clear breach of the no-advertising principle, even if it was within the rules, yet it was allowed to continue. Indeed the practice was encouraged, with a website offering sponsorship opportunities and carrying case studies, such as HSBC's link with Saving Planet Earth. That site was suspended only after The Times highlighted its existence.

It was only the determination of ITV, in alliance with the Radio Centre, representing commercial radio, that got the practice stopped. However, to get the BBC Trust to intervene, they first had to complain to the BBC's Fair Trading and Editorial Complaints units, both of which found in favour of having Robinsons as a sponsor. An ITV insider said: “The arguments they were using were so technical - angels dancing on a pinhead - to justify their position, it was incredible.”

ITV and the Radio Centre had to go through several months of BBC bureaucracy before they could complain to the BBC Trust. Although BBC executives said that sponsorship was within the rules, the Trust concluded something quite different. “UK audiences expect to receive BBC programmes that are free of advertising and wholly impartial,” it said, in a simple statement of principle that those seeking sponsors seem to have missed.

Sports Personality of the Year “compromised the editorial integrity of the BBC by giving the impression that [via a programme] part of a BBC service had been sponsored”, the Trust said. The Robinsons logo was too prominently displayed to viewers on the programme set, and the number of mentions of the brand - four on BBC One and three on Radio 5 Live - was “not editorially justified”, it said.

Opposition politicians grasped the seriousness of the lapseJeremy Hunt, the Conservative Shadow Culture Secretary, said: “This is a slap in the face for the BBC, and rightly so. When commercial broadcasters are fighting to keep their heads above water, the BBC should know better than to pile on the misery by sucking in sponsorship money through the back door.”

Finally Mark Thompson, Director-General, grasped the point of principle, and decided that all programme sponsorship should end, once existing contracts are honoured - since even the BBC Trust had ruled that the activity was technically within the rules. What is not so clear, though, is why anybody felt it to be acceptable for the BBC to seek money from advertisers in the form of sponsorship for so long, and why so much effort went into defending the activity now deemed to have harmed the corporation's integrity.

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