Asset growth in the robo-advice market has been driven by rising markets rather than new customers "suddenly seeing the light", according to a report by Boring Money.
Assets under administration in the robo-advice grew by 19.7 per cent during 2017 but this compared to FTSE 100 growth of 11.9 per cent.
Customer numbers have remained "largely static" while the average account size has more than doubled from £25,000 five years ago to £53,000 in 2017.
Holly Mackay, managing director of Boring Money, said: "We can see the market has grown. But this is not earth-shattering, tipping point stuff.
"The source of this growth is now however millions of new customers who are suddenly seeing the light. It is largely a factor of increasing account balances."
She said the exception to the online investment providers struggling to acquire new clients were companies like Hargreaves Lansdown and Nutmeg.
Hargreaves Lansdown continued to dominate the non-advised investment market with its market share increasingly very marginally to 42.1 per cent.
Interactive Investor’s acquisition of TD Direct Investing and Trustnet Direct made it the second largest player in the market with a market share of 9.8 per cent, with Fidelity following in third spot with assets under administration of £16.9bn.
During 2017 there were 11 new entrants into the online investment space but market share remained concentrated among the biggest companies.
The top five companies had a 72 per cent market share, according to Boring Money's research.
Ms Mackay said: "The board game that is online investing in 2018 is struggling to attract new players.
"If we are really to reach out to those suspicious savers and grow this market, we need to tackle the rules of engagement, design of the board and the way in which we market the experience."