Saying there should be fewer charity shops is viewed a bit like taking sweets from children or not giving up your seat to an old lady on the bus. But Mary Portas is right that too many of them can be a bad thing.

Speaking before the Parliamentary Group on town centres last week, the government’s high street adviser said that there should be a cap on the number of charity shops on individual high streets, and that the 80% rate relief they receive should be offered to start-up retail businesses as well.

Expressing views like these is always going to be controversial. Charity shops and the volunteers who work in them do an amazing job for very worthwhile causes, and the best ones have retail standards every bit as good as many ‘professional’ retailers.

As part of the mix on many secondary and tertiary high streets they really add to the retail offer. The problem is that when the market is tough, there can be too many of them. Now that landlords have to pay rates on empty properties, it’s far more attractive to put a charity shop in than leave it empty or persist with efforts to find a commercial tenant who might find the full rates burden prohibitive.

Once there are more than a handful of charity shops on a high street, they start being a blight. The quality of goods they sell inevitably becomes lower, as they’re shared among more shops, and the high street becomes less of a destination, as shops selling secondhand product are never going to be a primary driver of footfall.

With rates playing an ever greater part in the total occupational cost of property, the playing field should be levelled to help start up retail businesses benefit from the same perks - especially as in some cases now charity shops are selling new product not secondhand putting them in direct competition with mainstream retailers. And - while I’m not sure how it would work in practice - Portas’ idea of a cap on the number of charity shops is a good idea and the sort of bold thinking which is needed if she is going to succeed in revitalising our high streets.