The Financial Conduct Authority has said clients "orphaned" by advisers can end up paying more for assets which remain on the platform their adviser used.
In the interim report of its platform market study, published today, the regulator estimated there were currently more than 400,000 such client accounts - around 10 per cent of the total - with more than £10bn of assets.
The FCA also warned this problem was increasing, with a 9 per cent increase in the number of orphaned client accounts between 2016 and 2017.
Some adviser platforms impose extra platform fees on orphan clients, of up to 0.5 per cent in addition to their pre-existing platform charges.
The FCA estimated around 10,000 orphan clients were currently paying extra fees amounting to over £1.2m every year.
The regulator said: "We have two main concerns. Orphan clients have limited ability to access and alter their investments on an adviser platform so are paying for functionality that they cannot use.
"While many platforms told us that they encourage orphan clients to find a new adviser or switch to a [direct-to-consumer] platform, some platforms also charge orphan clients extra fees, of up to 0.5 per cent on top of their pre-existing platform charges."
The FCA has said it is now considering measures to tackle price discrimination between orphan and existing clients.
It is also considering whether to require platforms to have a process in place to get these customers to switch to a more appropriate proposition.
One of the problems the FCA encountered was that, with one exception, platforms do not actively monitor whether there has been activity on accounts with ongoing advice charges.
This means where there has been no activity for some time, sometimes for more than a year, and platforms are not confirming clients are receiving an advice service, this may mean platforms are enabling advice fees to be collected from clients who are not getting an ongoing advice service.
To address this the FCA is considering whether to impose a requirement on platforms to check, if there is no activity after a year, that their customers are receiving an advice service, and inform the FCA of orphan clients who are still paying an adviser for advice they no longer receive.