Some 3.6 million Santander customers will be hit by fee increases on the bank's flagship 123 range.From 11 January, the charge for the 123 current account will rise to £5 a month from £2, and monthly fees on the credit card will be hiked 50% to £3 a month.The cashback offers on the current account and credit card will not change for existing customers. However, credit card cashback will be capped for new customers. The 123 credit card pays 1% cashback on supermarket shopping, 2% on spending in department stores at 3% on transport. From January, each element will be capped at £3 a month.According to Santander's latest annual report, some 3.6 million people in the UK held at least one 123 product in 2014 and Santander increased its current account users by 47% over the year. Nicolas Frankcom, money expert at uSwitch.com , says: “An almost threefold increase in fees is a huge kick in the teeth for Santander 123 customers. Many will feel cheated by Santander moving the goalposts, and those drawn in by tempting cashback offers will have to spend more on household bills just to break even. However, it's worth noting that Santander still pays a respectable 3% interest on balances between £3,000 and £20,000." Moneywise VerdictThe current account is now much less attractive for smaller balances. Interest on deposits of less than £3,000 won't cover the monthly fees, even for people who don't pay tax on their interest.The cashback scheme remains attractive and people with large balances will still get plenty value from the account. Someone with the optimal £20,000 balance will get £540 interest before tax, after paying fees.