The deal will give the US arm of Grant Thornton funding to make targeted investments in staff, technology and infrastructure, and is set to be finalised by the end of June. It will not affect the UK business.

New York based New Mountain Capital LLC is a growth-oriented investment firm with approximately $50bn (£40bn) in assets under management.

The transaction is subject to US regulatory approval and other standard closing conditions. Following the closing of the transaction, which is expected in the second quarter of 2024, Grant Thornton will operate in an alternative practice structure: Grant Thornton LLP, a licensed CPA firm, will provide audit and accounting services, and Grant Thornton Advisors LLC will provide business advisory and non-attest services.