That probably everyone knew about except me!
Noticed last week a £10 payment I didn't recognise go out of my bank account to "shopper discounts and rewards". Googling it, it took me to moneysavingexpert where there are approx 36 pages of people being taken in by this kind of legit scam.
Mine was traced back to a pop up ad on the Ryanair site when I last booked a flight. I vaguely remember something which looked like it belonged to Ryanair asking me if I wanted a £15 voucher off my next flight. If so I had to subscribe to this shopper discount thing. No charge was specified (or I'd have looked into it obviously) and I thought no more about it. Apparently, the websites allow this company to have your bank details and you suddenly find yourself monthly debited for £10 membership of this discount thing.
I cancelled my "membership" straightaway. Apparently one person had been debited for almost 2 yrs before he noticed, but once he phoned them up he got his money back straightaway.
Biggest culprits for allowing your details to be accessed are apparently Ryanair, Easyjet and Pizzahut. There was a list of others on moneysavingexpert.