Magnus Wheatley, managing director of Charles Stanley Direct, questioned the validity of the research.
He said: "Cutting data to suit individual platform pricing is fraught with pitfalls. No two investors are alike and savvy clients select their platforms based on the features that different platforms offer."
Mr Wheatley said Charles Stanley waived the platform fee for clients holding stocks and shares in their Isa if they did one chargeable trade per month, which then amounted to a total charge of £138 per year.
"A client could hold several million pounds in an account and only be charged this sum," he said. He added the platform had a loyalty scheme which "aggressively" cut rates.
Danny Cox, head of communications at Hargreaves Lansdown, agreed investors needed to choose the provider that offered the best value for them, based on their individual requirements.
He said: "Our clients value the excellent service, investment tools and research, the extensive choices, the full range of tax wrappers, the mobile app and now the new active savings service."
Jason Hollands, managing director of business, development and communications at Tilney Investment Management, which owns Bestinvest, said different fee structures worked well or less so dependent on the level of assets held.
He said: "Fixed fee structures or those with minimum thresholds can be particularly costly for investors with more modest amounts to invest – like the vast majority of the public who are actually on average earnings."
Fidelity did not want to comment.
Alan Chan, director and chartered financial planner at IFS Wealth & Pensions, said charges were an important consideration when choosing an investment platform as they acted as a drag on investment returns.
However, he added they were not the only consideration for an investor.
He said: "It is the same reasons why we don't all shop at Lidl. Other factors like fund choice, fund share classes available, ease of access, financial stability and so on also play an important role.
"For instance you may find a platform that is cheaper but it might not have the fund you want to invest in or, if it does, you cannot invest in the lowest cost share class.
"Alternatively, it could simply be that the platform does not have a smartphone app to monitor your investments or customer service is very poor. It’s important to consider the whole package of what you are getting for your money."
Dippy Singh is a freelance reporter for FTAdviser