A new breed of payday loan brokers are making as many as 1m attempts per month to raid the bank accounts of some of the poorest members of society.

The behaviour is provoking alarm at one of Britain’s biggest high street banks, Natwest, which says it is being inundated with complaints from its most vulnerable customers.

NatWest said it is seeing as many as 640 complaints a day from customers who say that sums, usually in the range of £50 to £75, have been taken from their accounts by companies they do not recognise but are in fact payday loan brokers.

The brokers are websites that promise to find loans, but are not lenders themselves. Often buried in the small print is a clause allowing the payday broker to charge £50 to £75 to find the person a loan – on top of an annual interest charge as high as 3,000%. In the worst cases, the site shares the person’s bank details with as many as 200 other companies, which then also attempt to levy charges against the individual.

The City regulator has received a dossier of information about the escalating problem, and the Financial Ombudsman Service also confirmed that it is facing a wave of complaints about the issue.

NatWest, which is owned by the Royal Bank of Scotland, gave as an example a 41-year-old shop assistant who took a payday loan of £100 at 2,216% interest. A month later she complained to NatWest after seeing a separate fee of £67.88 paid to My Loan Now and £67.95 to Loans Direct on her account, companies she said she had never dealt with.

The broker sites tell customers they need their bank account details to search for a loan, but then pass them on to as many as 200 other brokers and lenders, which then seek to extract fees, even if they have not supplied a loan. The small print allowing the site to pass on the details and demand payments can be hidden in the site’s ‘privacy policy’ or in small print at the bottom of the page.

The sites use sophisticated methods to take money from personal bank accounts. They typically push their charges through bank payment processing systems between midnight and 3am, knowing that state benefit payments are added to accounts just after midnight. When the person living on unemployment or disability benefit wakes in the morning, they find their money has already vanished.

RBS Natwest, whose parent is majority-owned by the taxpayer, said it has terminated payment arrangements with 20 payday loan brokers already, but is battling against sites which reappear under various .net or .uk domains.

Terry Lawson, head of fraud and chargeback operations for RBS and NatWest, said: “We’ve seen large numbers of customers incurring charges they don’t expect when using a payday loan broker since July this year. Customers’ account or debit card details are gathered and sent on to up to 200 other brokers and lenders who charge them fees for a loan application.

“At its height we were seeing up to 640 calls a day on unexpected fees, but we’re pleased to say we’re seeing this decrease on account of the actions we’re taking to help stop these sharp practices.”

Wendy Scurr from Middlesborough, who lives on disability benefits, looked for a loan online to buy a new settee. “I put in my bank details as they said I had got the loan. But as soon as I submitted the final bit of information, it popped up that I had been declined. I felt that I had been conned out of my bank details, but I thought not much more about it.

“But on the Friday when I went to take some money out I found there had been two payments made of £67.88 to My Loan Now and £59.99 [to another lender].

“I went into the bank and they told me that six minutes after My Loan Now had taken the £67.88, it attempted to take the money again but as I had nothing left it was rejected.” She has since had to change her bank account to stop repeated attempts to take money, while on her phone she receives as many as 20 or 30 calls and texts a day from payday loan brokers.

My Loan Now’s website displays a warning that it will charge a “one-off loan matching fee” of £67.88.

NatWest said that during August it saw 1m attempts by payday loan brokers to take money from its customer accounts, although the majority were rejected as the customers were already seriously overdrawn. It added that it is working with the “merchant acquirers” – such as WorldPay and Barclaycard - to blacklist the brokers where possible, and from next month will entirely block payments to two of the major players.

“We are reaching out to customers to warn them of these fees and taking steps to block the transactions altogether.

We are also actively working with the industry to raise awareness of these practices and in many cases halt some of the brokers’ operations, but, these are sophisticated organisations, they are resourceful and more needs to be done at an industry and regulator level to protect customers who may already be in vulnerable situations. If one of our customers finds they have paid these charges, they should get in touch , so that we can stop payment of further charges and help them recoup any funds already paid, if possible”.

What shocks many of the victims of payday loan brokers is that the companies are usually authorised by the Financial Conduct Authority. The FCA said it has only recently taken on the job of authorising credit brokers, which was previously handled by the Office of Fair Trading. What is called “interim authorisation” was granted to 5,247 brokers, and only since 1 October has the authority begun assessing applications in detail.

My Loan Now is the trading name of Katsea Financial Solutions, which gives its address in Ipswich and runs ten other loan brands.Peter Tuvey, a director of Katsea, told the Guardian he did not charge fees and that My Loan Now was an Isle of Man-registered company that had no connection to him. But a check on the Isle of Man registry showed Tuvey was also registered there as a director of My Loan Now.

Tuvey said: “I resigned as a director of Katsea (Isle Of Man) in June 2014. My resignation was due to the company changing its principal business practices from a free comparison site to other practices.” He did not respond to further requests for information.

Loans Direct is run by Syed Rizvi from an address in Manchester. Neither Loans Direct nor Rizvi could be reached for comment.