In an interim trading statement today, the Bristol-based firm said it had been notified of a major levy increase by the FSCS due to a rise in the FSCS 2020/21 budget.
This could mean a levy of between £12.5m and £13m, it said.
Despite the concerns of a much higher levy and the impact of Coronavirus, the firm reported buoyant figures for the four months ended on 30 April and has declined to use government support.
The company saw 94,000 net new clients in the period pushing the total to 1.368m. An advertising and marketing campaign this year, particularly focused on London and the South East, boosted numbers.
It said many of the extra clients were relatively new to investing.
Year to date net new business was £6.3bn and year to date total revenue was £448.1m, up 13%.
Net inflows were £6.3 billion compared to £5.4bn in 2019 and revenue for the period was £190.2 million (2019: £159.5m).
The board plans to maintain its dividend policy for the 2020 financial year.
The Coronavirus caused huge volatility in March and April, the firm said, but significantly higher stock broking revenues were driven by record dealing activity. March and April both saw new daily dealing records.
Chief executive Chris Hill said: "During this exceptionally volatile and challenging period, Hargreaves Lansdown has performed strongly, adding 94,000 net new clients and £4 billion of net new business.
“We have and continue to be focused on the health and well-being of our colleagues and their families. We are not seeking government assistance, nor are we furloughing any employees or enacting any redundancy programmes.
“There remains much uncertainty in the coming months and hence, like many businesses, we cannot predict levels of new business or client activity. However, we are confident that the strategy we have invested in, with our focus on the needs of UK investors and savers and delivering the highest level of client service, means that we are well positioned to deliver continued attractive long-term growth."