I was almost arrested in Tesco this week. My crime? Comparing prices. Evidently, this is such a security issue for Tesco that it wants you booted out of the store. The deputy manager rushed up to me within minutes of my arriving at one of its London supermarkets. The security cameras had spotted me with a pen and paper in hand, noting the prices of goods on the shelves. "Excuse me, what are you doing?" he said. I told him I was, well, writing down prices.

"You're not allowed to do that. It's illegal. Where are you from? Are you from the media?" I don't feel Tesco has any right to demand my employment status, so I just said: "I'm a private individual, I'm buying some stuff here, and I'm comparing prices."

It obviously didn't satisfy him. "It's illegal to write things down and you can't take any photographs, either. If you want to check the prices, take the item to the till and pay for it there. The price will be on the receipt," he said, pointing me to the exit.

A store manager turned up, while another Tesco employee in a suit hovered in the background. "He's writing down prices," the deputy said to his superior, identifying a practice that evidently brings the bosses out in force.

I asked the manager if there had been a law passed which made it illegal to write down Tesco prices.

"Look, it's company policy, you're not allowed to do it," he said, perhaps accepting that Tesco doesn't actually write the laws of the United Kingdom – well, not yet at any rate. I showed him my notebook. Scribbled down were prices for Highland Spring sparkling water on the shelves right in front of me. Three 1.5 litre bottles were £2 (66p a bottle). On the shelf below, a pack of four identical bottles were £3.08 (77p a bottle). In other words, buying in bulk was a worse deal. Can you explain that? I asked. "It's an offer, innit? There's lots of offers in the store. Why do you want to know?" At this point I volunteered that I was doing "market research".

I said I would continue to look around the store at prices, using my eyes only. Would Tesco object to me using my eyes? At this point they left me alone, but, before buying my goods and leaving, I felt I was being followed. Goodness knows what would have happened if I had tried to take a photograph. Perhaps Tesco would have pushed for a custodial sentence.

But there's a serious point here. Consumer journalists should be able to scrutinise prices without being harassed. How else can we be sure of the veracity of "half price" offers? And should it be the case that security staff can throw me out for taking a photograph of a bottle of Highland Spring?

If we on Guardian Money bought every item available in Tesco, Sainsbury's, Morrisons or Asda every day, we would be able to check prices, and see if that bottle of wine, or those washing powder tablets, really are half price. But we can't, and trading standards officers don't have the resources to, either. Spot checks are about the best we can do.

But even that, it appears, is unacceptable to the likes of Tesco. Next time I'll wander round the store speaking the prices into my mobile phone. It's got a record function. Note to Tesco company policy writer: ban customers from speaking into their phones.