Students using an exam revision app say they have been charged repeatedly, even when they no longer need it or have not been able to use it.
Revision App claims to offer effective online revision through its website or phone app.
A BBC investigation has found the company has taken people's money then failed to respond to their complaints.
Its founder said the cases were a "poor representation of the experience customers actually encounter".
Entrepreneur of the year
Revision App offers animated videos and quizzes to help with exam revision. It was set up in 2011 by Jermaine Hagan.
In 2013 he was named as Shell's young entrepreneur of the year and appeared on stage at the Conservative Party conference to explain how a government-backed start-up loan had helped get his business off the ground.Image caption Jermaine Hagan addressing the Conservative Party conference in 2013
"I've been able to help one million young people learn on their mobile devices," he told the audience, which included the then chancellor, George Osborne, and education secretary, Michael Gove.
"In the next five years I want everyone to know about Revision App. We want to be huge," he said.
The business got positive coverage in national newspapers, the BBC News website and glowing reviews online. But by 2015, complaints had started to appear. People were posting reviews saying they had tried to cancel, but could not, and were still being charged a monthly fee.
Among them was Jane Finch, from Caerphilly, who had signed up to Revision App to help get her son through his GCSEs. She had been happy with the service until the time came to get rid of it.
"After his exams I thought oh well he's done with this so I emailed them to cancel it," she told BBC Radio 4's You and Yours. "I got an email back saying we'll contact you within three working days but I never had any response from them."
There is no address or telephone number on the Revision App website so customers can only email the company. When Jane Finch's emails continued to go unanswered, she contacted her bank and got them to block any further payments. By then, another £19.99 had been taken from her bank account for a month's access she did not need.
Other customers have paid for the service but have not been able to use it at all. In January 2017, Jo Hinds, from Cardiff, found Revision App while searching online for something to help with her daughter's AS-level exam revision. She paid £29.99 to access the website's content and received an email confirming her order.
"That email informed us that within the next five minutes we'd receive our member details and access instructions. We are still waiting for those," she said.
Without them, Mrs Hinds's daughter has been unable to access the content on Revision App's website. Mrs Hinds complained but her emails also went unanswered. She thought she had paid a one-off fee so was alarmed to discover she had been automatically signed up for rolling monthly payments - at least, that was what the email confirming the order told her.
"There was no mention of that at the stage of payment," she said. Her bank refunded her money and blocked any possible further payments.
"1 day sale"
The Revision App website includes the logos of four major exam boards, but they have all told the BBC that they have nothing to do with the service and their logos have been used without their permission.
Another discrepancy is a promised discount on access to GCSE videos. The website calls it a "1 day sale" yet it has appeared on consecutive days over 24 hours apart.Image copyright Revision App Image caption Revision App's '1 day sale' appeared on consecutive days, 24 hours apart.
Alongside the complaints, positive reviews of Revision App have appeared on online forums. But when things have gone wrong, why have customers' complaints gone unanswered? It is still not clear.
When contacted by You & Yours, the founder of Revision App Global Ltd, Jermaine Hagan, put customers' complaints down to "temporary errors" with the software used to give out access codes and process cancellations.
He said both problems had been fixed "quite some time ago" but failed to explain why, in that case, complaints have been made during 2015, 2016 and now 2017. He said access to A-level content was not currently provided - so why was Jo Hinds's order processed at all? And why was A-level content still being advertised on his website?
Mr Hagan failed to provide any further clarification. In an email he stated the cases highlighted "are a poor representation of the experience customers actually encounter when learning with us".
Listen to BBC Radio 4's You & Yours report on Revision App.