In this celebrity-obsessed age it is extraordinary just how few charities use celebrities in their fundraising advertising. Perhaps they worry that some tabloid scandal will blow up and reflect badly on them.

It’s a fact, though. A recent analysis of 189 assorted ads, loose inserts and mail shots found only six examples in which famous people were used. Two of them are shown here: one is an ad for the Refugee Council starring Colin Firth, with his shirt on for once, and one for the Prostate Cancer Charity with Neil Fox.

They demonstrate very well some of the problems posed by such ads. First, when is a star not a star? Answer: when you don’t know him from Adam. Even this decrepit writer had no difficulty in recognising Colin Firth, but Neil Fox? Who he? Apparently he is a of player of discs on the wireless in the UK and has been a judge on some talent show or other.

On the other hand, at least the prostate ad has the merit of a message that links the person featured to the cause and could conceivably work to some extent, whether you knew – or cared – who Neil Fox was. And there’s no doubting that it’s a pretty striking portrait – with its stark contrast of light and shade and eyes staring straight at the reader – which could attract attention.

In contrast, the Colin Firth ad suffers from being just that – a Colin Firth ad. He has just been grafted on to the message with no obvious connection or reason other than his fame. It’s as if somebody said, ‘we’ve got this fantastic star, what the hell do we do with him?’ It would have been better to show the star visiting an asylum centre or being otherwise involved with the charity so that the ad was about something other than Colin Firth – even a first person quote would have helped. The best way of using celebrities is as an add-on endorsement giving reassurance and authority.

In their loose inserts, Plan International consistently get the balance right by concentrating on their core message and adding a celeb on the back page (they’ve used Timothy Spall, Twiggy, Joanne Harris and Fiona Phillips among others) to sprinkle some stardust and it works well.

Flora & Fauna International have used Sir David Attenborough – so close to secular sainthood nothing could possibly go wrong.