Interactive Investor is to migrate Alliance Trust's direct customers onto its platform as the purchase of the rival platform completes, but plans are uncertain for its adviser clients.
The company has already received regulatory approval for its acquisition of the Alliance Trust savings platform.
Richard Wilson, chief executive of Interactive Investor, said: "When it comes to the advised and partnership part of the ATS business, we have been talking and listening.
"With the keys to the door, we can start to lay down plans. Meanwhile, advisers and partners should expect business as usual."
The acquisition brings together retail investment platforms with combined assets under administration of £36bn and about 400,000 customers.
The deal was first announced in October 2018, with Interactive Investor paying £40m for the assets of the platform, including the latter’s office in Dundee.
Alliance Trust Savings had previously been owned by Alliance Trust, a £2.9bn investment trust that primarily invests in global equities, but also operated the platform.
Both platforms charge clients flat fees, rather than a percentage of assets, a policy that Interactive Investor stated will continue.
The platform is owned by the US private equity firm JC Flowers and the acquisition has been funded by Interactive Investor’s own cash, rather than external debt.
Mr Wilson added: "The completion of the ATS acquisition is an important milestone.
"It cements our position as the market challenger in the UK direct-to-consumer marketplace and adds essential scale to support the on-going technology and talent investment needed to drive our customers’ service experience ever higher."
Lord Smith of Kelvin, Alliance Trust’s chairman, added: "The board is pleased that the sale of ATS to ii has completed.
"The sale completes our transformation into a fully focused global equity trust, which should improve returns to shareholders."