This weeks' announcement of Tesco's bumper profits highlights how powerful supermarkets have become - and should set alarm bells ringing.

BE afraid. Be very afraid. So afraid, you will totally change the way you shop. This week, Tesco announced profits of £2.03 bn, up 20 per cent on last year. It accounts for one pound in three spent in British supermarkets and its sales of clothes, electrical products and CDs have hit other businesses in the high street. It's expanding all over the world.

Time to think about the power supermarkets have over our lives.

Long before Jamie Oliver came along to tell us what muck we were feeding our children, Joanna Blythman - equally knowledgeable, equally passionate, but with a more elegant turn of phrase - was hammering home the same message. In particular, she was warning us about supermarkets.

We think supermarkets offer us choice. Not at all, says Blythman. "Supermarkets sell us what it suits them to sell. They decide what makes them money and then market it to us so that we want to buy it," she says.

And we dutifully push our trolleys up and down the aisles.

Supermarkets have amazing powers. From being simply useful additions to the high street, they have now taken it over. They now have virtual control over what we want and how much we pay for it. They work their way round planning laws and squeeze their suppliers hard. Their tentacles reach all over the globe.

Much of what Blythman tells us in Shopped, her expos of the power of British supermarkets, we already knew, or guessed - or had seen with our own eyes. And yet, somehow, so blinded are we by the dazzling lights and enticing displays of the supermarkets, that we've managed to ignore it. Here are a few reminders:

* Despite their advertising claims, supermarkets are often much more expensive than independent shops and markets.

l They like to sell us ready meals and processed foods - often loaded with unhealthy amounts of salt, fat and sugar - because there's more chance of a bigger profit. Supermarkets need food that looks good and stays fresh. Taste is a minor matter.

l Supermarkets kill town centres. Butchers, bakers and fishmongers are closing at the rate of 50 a week. If current trends continue, there might not be a single independent food store left in the country by 2050.

l Money spent in local shops boosts the local economy. Money spent in supermarkets goes mainly to boost their enormous profits.

l The younger generation of shoppers, who have only ever shopped in supermarkets, have no idea of when foods are in season or what they're really meant to taste like.

l British consumers spend three times more than any other country in Europe on ready meals. Chef Rowley Leigh estimated that a supermarket pasta bake costing £1.99, would cost only 40p to make at home. A beef casserole costing £5.58 would cost only £1.50.

l Despite the striped aprons and straw boaters, most supermarket butchers are not butchers. They sell only selected cuts of meat and haven't much clue what to do with it. Meat is not hung long enough to develop flavour as that takes too long and costs too much.

l Supermarkets now also offer loans, credit cards, pharmacies, clothes, electrical goods, insurance, eye tests, jewellery... Before long, there will be no need for a high street - supermarkets will have taken over the world.

So why do we shop there? Because it's easy. The single most important reason is probably to do with car parking. Try doing your weekly shopping in Darlington town centre, for instance, and you would have to pay for parking and walk quite a distance back to the car. Pop into Sainsbury's and you can park easily and push your shopping right to the car. Of course it's a lot easier.

It's quicker too. And for many of us, time is as precious as money.

Darlington market is a treat for shopping - and if you could get one hour's parking right next to it, it would be as successful as it deserves. But you can't, so it's not.

Surely it's no coincidence that Northallerton High Street flourishes, with butchers, bakers and a top class grocer - and it also has free parking. Apart from at the busiest times, it is often possible to nip to Northallerton, park within a few yards of the shop you want and be away again in 20 minutes, without going near Tesco or Safeway.

Yes, of course we all need a supermarket sometimes, but not all the time, not for everything.

We can try to do more of our shopping in local businesses - at farmers' markets, where you will get locally grown and produced food that actually tastes of something; at independent butchers, where you will get help and advice on cuts of meat; at small shops and stalls, where owners know and care about what they're selling.

Too many of our town centres are being left to charity shops and little else. The big supermarkets already have more than 80 per cent of our food shopping. If we're not careful, they will soon have it all.

And what choice will we be left with then? And how much will it cost us?

* Shopped, the Shocking Power of British Supermarkets by Joanna Blythman (Harper Perennial, £7.99)