A Liverpool-headquartered investment management company asked for restrictions to be placed on its activities following the "loss of several experienced staff who cannot easily be replaced".
Blankstone Sington contacted the Financial Conduct Authority (FCA) after the departures "caused difficulties for the company to provide its normal standard of service".
The business said it is currently in the process of recruiting replacement staff.
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Among the departures was chief executive Neil Turner who left his role in the summer to become investment director at Investec.
Under the voluntary agreement the firm, which is a broker for Everton FC, is restricted from disposing of its assets without the written consent of the FCA.
It is also restricted from accepting client money and custody assets from existing clients, and from opening accounts for new clients without permission.
Blankstone Sington added that it is in "regular dialogue" with the FCA and external advisers to "reach a successful outcome to the situation".
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However, it also said it is unable to confirm when the restrictions will be lifted.
According to its most recent accounts, for the 12 months to the end of May 2021, the company had funds under management of £472.4m.
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Interim chief executive Ben Taxman said: "We have implemented these voluntary restrictions in the best interest of our existing clients who will remain unimpacted from a business perspective in respect of Blankstone Sington’s ability to continue to provide them with normal personal and professional services to a high standard.
"The restrictions applied relate to the opening of new client accounts and the transfer in of new client funds only.
"Existing clients’ money and investments remain safe and continue to be held in accordance with FCA requirements.
"This measure, which is anticipated to be short-term only, is part of a plan to address temporary resource issues related to the departure of some experienced staff.
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"Blankstone Sington have been proactive in already securing a number of highly experienced individuals including a renowned expert with immediate effect, currently focussed on driving necessary strategic transformations aimed at alleviating any concerns and ensuring the removal of restrictions as soon as is practicable and in the interests of our clients."
A statement posted on its website said: "Blankstone Sington has requested of the Financial Conduct Authority (FCA) a voluntary agreement (VREQ) to restrict how it carries out its investment business.
"The board has made the request because of a loss of several experienced staff who cannot easily be replaced.
"This has caused difficulties for the company to provide its normal standard of service. The company is actively recruiting for replacement staff.
"The voluntary restrictions took effect on 16th November 2021.
"Our clients’ money and assets remain safe and are being held in full accordance with the FCA’s rules.
"We are restricted from disposing of our assets without the written consent of the FCA.
"This requirement does not apply to disposals made during the ordinary course of our business.
"For example, we are not restricted from making our usual payments to our suppliers and our salary payments to our employees.
"The requirements also restrict us from accepting client money and custody assets from existing clients, and from opening accounts for new clients, without the written consent of the FCA.
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"There are exceptions from this requirement, for example movements associated with open trades and positions of our customers.
"The company is in regular dialogue with the FCA and external advisers to reach a successful outcome to the situation. At the moment we are unable to confirm when these restrictions will be lifted.
"As always, we remain committed to our clients and will keep you informed as the situation evolves. We would like to thank you all for your patience and understanding."
The FCA declined to comment.