Hargreaves Lansdown will acquire up to 16,000 new clients and £420m of assets having reached an agreement to acquire the savings scheme account holders of Witan.
In a statement to the stock market, Witan confirmed it was winding up its savings platform.
Bosses at Witan said their account holders would be better off transferred, free of exit charges, to a specialist investment platform.
Harry Henderson, chairman of the Witan Investment Trust, said the move was also made because there had been a substantial increase in the regulatory cost associated with the provision of the savings schemes.
Witan will stop administering the accounts in May when clients will be offered the option to transfer to Hargreaves Lansdown or pick another provider.
This is the seventh such deal reached by Hargreaves Lansdown in recent years, with the firm having previously acquired the savings plan clients of RIT, Legg Mason Investment Trusts, JP Morgan Investment Trusts, Jupiter Investment Trusts, BlackRock Investment Trusts, Old Mutual Global Investors.
Chris Hill, chief executive of Hargreaves Lansdown, said: "Investors now expect to be able to access and manage their accounts on their mobile phones and tablets, on the move, in real time.
"More Hargreaves Lansdown clients log into their accounts via a mobile device than a traditional PC.
"As one of the largest supporters of investment trusts in the UK we are pleased to support Witan's decision and look forward to welcoming these new clients to Hargreaves Lansdown."