Optimising For Trademark Terms?

[Fruit & Veg]

I've got a client who would love for me to optimise his site using his competitor trademark terms and names. I've told him this isn't cricket and shouldn't be done plus he can do it to his hearts content on Google Adwords, but he now wants hard evidence of why this can't be done.

I've tried searching for hard evidence of the illegality or plain inhumanity of doing natural optimisation for competitor trademarks but have come up short. Can anyone point me in the right direction?

I've found a couple of US based cases but nothing in the UK.

[getvisible]

it can get you in legal hot water as don't forget your competitors' trademarks are their property.

It's fair to use them to review their product or service or to use it within context. But to "free-ride" on the back of their brand is something you shouldn't consciencely set out to do.