Hargreaves Lansdown has reached an agreement with JP Morgan Asset Management to transfer up to 33,000 Isa clients to its books with assets of £765m.
The clients own Isa accounts administered by JP Morgan Asset Management and under the terms of the agreement can transfer their holdings to a Hargreaves Lansdown Isa account.
Clients can also choose to move to another Isa provider if they wish.
This is the second transaction of this kind between the firms, the previous one having occurred in 2015.
Hargreaves Lansdown has made a practice of acquiring client banks from asset management firms in recent years.
Agreements were also reached with BlackRock Investment Trusts, J.P. Morgan Asset Management, Jupiter Investment Trusts, Legg Mason ISA, Old Mutual Global Investors, RIT Capital Partners, and Witan Investment Trust.
Clients who held their Isas directly with the asset managers were typically only permitted to invest in the funds run by those firms, but upon moving to Hargreaves, or another retail platform, they can invest in the whole of the market.
Hargreaves Lansdown will now write to the clients involved in the JP Morgan deal, offering them the chance to transfer to Hargreaves.
Clients who do not wish to do this have the option to transfer to another platform, but they cannot remain with JP Morgan.
Chris Hill, chief executive of Hargreaves Lansdown, said: "As the largest stocks and shares Isa provider, we continue to develop and improve our online and mobile services to meet the changing needs of the investor.
"We are pleased to support JP Morgan Asset Management’s decision and look forward to welcoming these new clients to the Hargreaves Lansdown service."