At the risk of over-simplifying, the law on comparative advertising goes something like this:
Trade mark law says that you can use a competitor's registered trade mark in a comparative ad provided that you don't imply that your product is associated with your competitor. This has seemingly been the case since 2007, when the Appeal Court ruled that Verimark's use of a BMW car in a TV ad for a car polish was kosher because there was no suggestion of any connection between the products (there is a possibility that our law will change following a recent European decision, but we'll have to wait and see).
Advertising law agrees, but imposes further requirements, for example the comparison made must be sustainable, and the ad must not be disparaging of the competitor.
The ASA recently dealt with a comparative ad where there was in fact no direct mention of the competitor. The ad was for Sunlight dishwashing liquid and it said that Sunlight 'washes twice as many dishes as its nearest competitor.' The ASA accepted that this was a reference to Ajax – not only is Ajax in fact Sunlight's nearest competitor, but the ad showed a bottle which looked remarkably like an Ajax bottle.*
The ASA also accepted that the claim was true (so remember which liquid to buy thrifty Scottish readers!). But it went on to find that the ad was disparaging and therefore contrary to the Code. Why? Because Sunlight dissed Ajax. Oooh! Turns out the ad was one of those deep, thought-provoking cinematographic masterpieces, so typical of this product sector - in one particularly poignant scene Sunlight lady refers to Ajax as 'that stuff', and in another pathos-filled moment Ajax chick looks a little disappointed with her choice of product. This, said the ASA, went beyond claiming a factual superiority to being disparaging.
Bit precious what! Surely a little robustness is part and parcel of advertising? What are you supposed to say - 'Your toothbrush is noticeably inferior to ours, but our respect for your product is undiminished!'
*It was the bottle that did it. In another recent ASA case involving arch rivals I & J and Sea Harvest, the ASA held that a reference by one company to Brand X was not sufficient to identify the competitor's product.