Research published yesterday by the UK consumer organisation Which? shows that regulations to prevent the advertising of junk food to children are not applicable to the TV programmes attracting the largest audiences of under-tens. This anomaly goes to the heart of the key moral question posed by such regulation, Ben Cooper writes, while the findings may well make further regulation more likely.

The question of how TV audiences should be assessed in terms of their suitability for the advertising of junk food arguably goes to the heart of the moral debate over such regulation - the choice between the protection of some and the restriction of others.

A report from UK consumer group Which?, published yesterday, suggests that regulations introduced by broadcasting watchdog Ofcom to control the advertising of junk food to children are ineffective because they are based on the proportion of children watching the programme rather than the absolute numbers.

A simple example would be represented by a comparison between The Simpsons, which includes some 163,200 children under the age of ten among its audience and is covered by the Ofcom restrictions, and the talent show The X-Factor, which has a total audience of 412,800 under-tens and is not, because the children watching represent a lower proportion of the much larger total viewing figure.

In fact, according to the Which? research, The Simpsons is the only programme in the top six, measured in terms of the total number of under-ten viewers, that is covered by the Ofcom restrictions. And it comes in sixth.

But the "proportion question" is not only significant in producing this apparent anomaly.

It could be argued - and indeed is argued by many - that the question of whether or not to regulate hinges on the balance between the importance of protecting those children who happen to be watching a mixed-audience programme from messages about less healthy foods, and the "freedom" of those adults watching to see such adverts - and of course the freedom of companies to be able to communicate with them.

Indeed, in many ways this question represents a clear representation of that moral dilemma which affects so many debates over regulation. The question of whether or not to regulate or how far regulation should go constantly comes back this question of the balance between defending civil liberties and protecting those who need to be protected.

However, advocates are often less concerned with debating such philosophical niceties, and more interested in absolutes. For Miranda Watson, food campaigner at Which?, it is simply a matter of numbers. The fact that five programmes with a higher under-ten audience than The Simpsons are not covered by the regulations represents an unacceptable flaw in the system.

"This research shows that unhealthy foods are being advertised during the programmes with the highest number of child viewers, such as The X-Factor," she says.

There is logic to the idea that one could simply base any prohibition on the number of children watching, effectively ignoring the "rights" of other people, on the basis that the protection of minors is more important. After all, that is what societies have always done with pornography, and few have any objection to that.

Once one accepts that the proportion of children is less significant than the actual number likely to be watching, it could then become a question of where one draws the line. However, Which? believes that the sensible option is to impose a time watershed of 9pm, which is exactly how we approach similar questions with regard to taste and decency.

As Watson continues: "The only way to shield children from TV ads for unhealthy foods is a 9pm watershed. There is no silver bullet in the fight against obesity but tougher restrictions on the way unhealthy foods are marketed to children will play an important role in helping parents to instil healthy eating habits in their children." Which?'s call was backed by the British Heart Foundation. The Food Standards Agency also supports a 9pm watershed.

Meanwhile, the Food and Drink Federation has called for more time to allow the current regulations to work, and draws attention to the fact that UK is already heavily regulated. It was an oblique reference to civil liberties but the industry's advocacy organisation would be unlikely - and probably unwise - to base a defence on the idea of defending the rights of adults to view advertising for unhealthy foods, particularly given that the Government has an almost equally pressing desire to reduce the consumption of such foods by adults.

The Advertising Association latched on to Watson's observation that there is "no silver bullet", suggesting that the 9pm watershed idea was a simplistic solution. This could be seen as rather a canny response given that Lady Peta Buscombe, chief executive of the Advertising Association, made direct reference to the recent report of the Government-backed Foresight Group which suggested that one single solution could not tackle the problem of rising obesity.

However, while the idea of a holistic approach to tackling such problems was the central thesis of that report, it was also generally in favour of enhanced regulation so it is unlikely that the defenders of the status quo would gain much succour from that quarter.

In fact, if the question of regulation does come down to a choice between protection and intervention on one side and freedom of choice on the other, there is a fairly strong indication that the administration of Gordon Brown will lean more significantly towards the former than that of his predecessor.

The Health Secretary, Alan Johnson, is expected to announce new measures to tackle rising child obesity as early as next month, according to some reports. The question now is whether the publication of the Which? research has made the inclusion of a 9pm watershed in the next raft of legislation more likely.