AJ Bell was the fast growing platform in UK on a percentage basis so far this year, according to data from Fundscape.
The company saw its platform's assets grow by 25.7 per cent, or £6.3bn, during 2017.
Meanwhile Hargreaves Lansdown saw its assets grow by the most in cash terms, by £12bn to £82bn this year, a 17 per cent increase.
That puts the FTSE 100 firm in second place in terms of assets under administration, behind Cofunds and ahead of Fidelity.
Cofunds has assets of £90.5bn, adding £6.9bn this year or 8.2 per cent, while Fidelty has total assets of £76.9bn but its growth didn't even make the top five.
Total platform assets across the UK increased by £19bn - or 3.6 per cent - to £560bn.
Bella Caridade-Ferreira, chief executive of Fundscape, said: “Despite Brexit and the regulatory headwinds facing the industry in the next few years, the platform industry is the de facto conduit for pension and investment solutions.
"As a result, platform growth is likely to continue in the same vein in the medium term.”
Behind Hargreaves Lansdown, Standard Life was the platform that showed the greatest level of asset growth, with £7bn added to its platform, an increase of 16 per cent.
Cofunds was also the most popular platform by sales in 2017 year to date, with gross sales of £17.5bn, compared to Hargreaves Lansdown’s £9.9bn.
Fundscape's research found there were now three platforms with more then £75bn in assets - Cofunds, Hargreaves Lansdown and Fidelity - together accounting for 45 per cent of industry assets.
Ms Caridade-Ferreira predicted that by 2022 platform assets would reach a total of £1.4trn.