Lloyds Banking Group and Scottish Widows will seek to terminate the investment management arrangements related to the assets to allow the review to take place.
In response, Standard Life Aberdeen's chief executives Keith Skeoch and Martin Gilbert said: "We are disappointed by this decision in the context of the strong performance and good service we have delivered for LBG, Scottish Widows and their customers".
This morning Lloyds ended six months of speculation about the funds, which belong to customers of its Scottish Widows insurance arm, by saying that it could no longer have them managed by a "material competitor".
Antonio Lorenzo, chief executive of Scottish Widows, said the company had begun an "in-depth assessment of the market" to identify replacement managers.
The Edinburgh-based company was managing the funds because Lloyds had sold the fund management...
Mr Gilbert said he was confident in Standard Life Aberdeen's future and dismissed concerns the business was going into reverse. However, while nearly a fifth of Standard Life Aberdeen's assets look like they might be walking out the door, this only equates to 5% of revenues, as these investment services are relatively low margin.
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The contracts have a 12-month notice period. The Lloyds contract represented a third of Aberdeen's assets.
The bank is launching its three-year strategy next week.
Khalaf says: "This would make some sense now the bank has recovered from the financial crisis and will be looking for opportunities to grow and diversify". That said, it only accounted for 5% of the firm's total revenue in 2017.
However, he says 12 months would be a very short amount of time to pull off such a move.
According to data from IPE's annual Top 400 Asset Managers report, Standard Life and Aberdeen ran a combined €393.8bn of institutional money at the end of 2016, making the merged entity the fifth largest institutional asset manager in Europe.