Nick Train, a long-term backer of Hargreaves Lansdown, has said the platform's assets could hit £240bn by 2020 - a figure which would mean adding £200bn in AUA over the next six years.
Hargreaves is now Train's top holding in his £750m Lindsell Train UK Equity fund, following a huge jump in price - shares are up 105% over the past 12 months - which has left some investors cautious on its future prospects.
On Wednesday the company's shares suffered a set-back, dropping 4% after it revealed its new pricing structure, but the upward trend remains firmly in place.
That is despite a major shift in its business model, caused by the Retail Distribution Review (RDR) and the move to ban rebates. As of 1 March Hargreaves will charge users an ongoing platform charge, as well as a host of other fees for various activities.
However, Train (pictured) said the business' continued potential to grow over the next half decade makes him reluctant to sell his holding, despite a share price - around £14.70 - he admitted seems high relative to current assets.
"The company is now valued at around 17% of its AUA. This is a very high percentage and in our opinion a reasonable rating would be 7.5%," he said.
"This means assets must grow [from £39bn as of October 2013] to £89bn for the company to look reasonably valued."
But Train noted AUA would hit some £240bn by 2020 if previous growth rates were sustained, a move he does not see as unfeasible.
"Assets have grown to today's short £40bn from £6bn since HL floated in 2007," he said.
"If that growth rate were sustained AUA would hit £240bn by 2020. This is not a prediction. However it does seem fair to ask - why not?"
"There seems no absurdity in suggesting HL could attain this sort of scale."
An asset base of £240bn would represent about 5% of the UK savings market, Train said.
The manager's £750m fund - which had an 8% position in the stock by the end of December - remains one of the top performing portfolios in the IMA UK All Companies sector, delivering 32.8% to investors last year, versus a sector average of 24%.
Developed by industry-wide group
Joined in 2002
'Educate clients' children'
Raised £15m earlier this week
From 8pm Friday 19 October