So why do you have to pay with cash or a card - why not pay on the phone?

Why not pay on your phone? To me the answer is four fold:

1. Somethings may not have scanned and the cashier may have to do it,

2. This is entirely under Sainsbury's control they don't have to involve anyone else to integrate payments,

while

3. They test consumer response. If they don't get consumer traction they haven't wasted money integrating payments for something consumers don't want.

And

4. To stop people walking out the store with a trolly full of booze that they haven't actually paid for.

@ Nick Green:

If the shopper does not have to unload his/her trolley (as stated in the article), your points 1 and 4 don not make sense?

As a regular Sainsburys customer, this is just as small evolution from in-store handheld scanners to using your own phone. If it means Sainsburys provides in-store wifi then great, at the moment I get absolutely no signal once I step inside the door of my local store ... Faraday cage effect?

Mobile payments - whether done by NFC or QR code or whatever - are always treated as CNP, thereby attracting higher MDF costs. This is true not just in this specific case but in all contexts. 'Walking out without scanning' - and hence paying - for something is a real risk with such mobile self-checkout systems. When I inquired with the provider of one such system about this point, I was told that shop-lifting happens even in cashier/till-based checkout systems and that it hasn't found any significant evidence that self-checkout poses a greater risk of shop-lifting. While the digerati might be satisfied with this reply, I personally found it very feeble and so might retail executives who have to bear the cost of pilferage.