Scottish banks are among the most expensive for overdrafts, and consumers are being bamboozled by opaque charging structures on current accounts.

That is the latest charge from consumer group Which? – though it has been refuted by the banks. It comes less than a month after the Financial Services Authority announced a crackdown on the sale of "packaged" bank accounts, and only six weeks after the Office of Fair Trading launched a new review of personal current accounts.

These developments have highlighted the need for customers to check they are not being ripped off by their banks and to switch elsewhere if necessary. But this is often easier said than done due to the complexity of bank charges.

Clydesdale Bank came out of the Which? survey as the worst offender for its high charges when customers go overdrawn without permission. Going overdrawn for two days a month will cost its Current Account Plus holders £900 a year.

As well as being charged interest at 29.99%, they also have to pay flat rate charges of £25 a month plus £25 a day if they overdraw by more than £25. For authorised overdrafts, the five highest charging accounts include the Bank of Scotland's Classic Account and Classic Account Vantage, and the Royal Bank's Select Account. Bank of Scotland charges 19.3% plus £5 a month, while the Royal Bank has an interest rate of 19.89%.

Overdraft interest rates have remained stubbornly high over the last 10 years, despite a fall in bank base rate from 4% to 0.5%. The average authorised overdraft rate has fallen by less than 3% from 16.56% to 13.74% according to Moneyfacts.

Defending its high charges, Clydesdale Bank spokesman Barry Gardner said: "All unplanned borrowing costs can be easily avoided by being aware of your account balance and any payments you have committed to make. Customers can access their balance within a few seconds, 24 hours a day, online, or over the phone. We constantly review our fees to ensure they remain appropriate."

Despite high levels of customer dissatisfaction, only around 4% to 6% of accounts are switched each year according to research group Defaqto.

A report by the Payments Council earlier this year found many customers lacked the confidence to switch accounts as they were concerned that something may go wrong or that providers may not adequately oversee the process.

Working out which accounts are most cost effective for overdrafts is very difficult because some providers charge a daily fee for an agreed overdraft, others a monthly fee, while some charge an interest rate and others a mixture of both. There may be also an additional fee for renewing an overdraft facility. Defaqto has called for "a simpler or consistent approach to overdraft charging."

More than 60% of packaged accounts offer a limited free overdraft facility.

However, these accounts come with an average monthly fee of £15.28, or more than £180 a year. They also include a variety of insurance policies and other features. The FSA calculates that one in five people now have these products.

However, in the past many people have been encouraged to upgrade on to these accounts even though they might include products they could not use, such as being too old to claim under a holiday insurance policy. From next April, consumers will have more protection because the FSA will require banks and building societies to check whether customers are eligible to claim under each policy.

It has been suggested there would be less mis-selling by banks if all current accounts were charged for, giving banks a secure income stream and dispelling the "myth" of free in-credit banking. However, Which? points out that even people with positive bank balances do not really get free banking. It says: "Consumers who stay in credit are also being hit through lost interest and hefty fees for withdrawing and spending cash abroad."

Peter Vicary-Smith, Which? chief executive, said: "The suggestion that banks should increase charges to avoid more scandals defies logic and is a slap in the face for consumers who are being hit hard by one of the worst financial crises in recent times."

More transparency would help. Michael Ossei, personal finance expert at uSwitch.com, said: "By sweeping the real cost of banking under the carpet, it makes it difficult for consumers to compare banks and work out which accounts offer the best value for their needs. At the moment consumers are stumping up millions in 'stealth' fees and charges which could be avoided if they were on the right account or provided with the right information."

The banks which receive the highest satisfaction ratings according to Which? are First Direct, Smile and the Co-operative Bank.

There were hopes that increased competition from the likes of Marks & Spencer, Tesco and Virgin Money might help to bring costs down but the new current accounts launched recently by M&S have disappointed in that respect. Both are fee-charging, packaged accounts, costing £15 and £20 a month. Bank charges and interest rates have not yet been revealed.

The Office of Fair Trading is not satisfied. In announcing its latest investigation, Claire Hart, OFT director, said: "We are concerned a lack of effective competition means the retail banking sector is not working in the interest of customers and businesses. We want to see banks become more customer-focused and this will be the central theme of our programme of work going forward."

The OFT plans to publish its personal current account review by the end of 2012.