Kleinwort Benson Investment Management and Thornton Unit Managers are being merged by parent company Dresdner Bank.
The two fund managers' names will be ditched as they join Dresdner's US-based RCM Capital Management.
The new group, Dresdner RCM Global Investors, will be run from the US and control more than £35bn in assets.
As part of the move, the number of unit trusts held by KBIM and Thornton will be cut from 33 to 24 and converted into Oeics next year.
Kleinwort Benson has £15bn funds under management, Thornton has £2bn and RCM has £18bn.
KBIM currently manages all the group's investment trusts, about 60 per cent of the UK-based unit trusts and 40 per cent of the offshore funds.
Dresdner RCM Global Investors head of European investment funds Mark Lynam says the merger is necessary to allow the company to compete successfully in the international market.
He says: "It has taken so long to merge the companies officially because it is such a big step into a global business and we wanted to get it right.
"There will be no redundancies. We will be integrating our investment teams so that key staff will continue to run funds while more junior staff will be involved in key specialist areas.
"Independent advisers and investors will be targeted with a wider selection of financial products to choose from as all three businesses complement each other."
Lynam says the new company plans to develop the European market with with what it claims is a unique grassroots investment technique pioneered by RCM.
Head of marketing Stephen Westwood says: "Looking forward towards the next 10 or 20 years, we had a choice of becoming a specific niche firm or big global player. The shear size and back-up we can now offer will enable us to take a leading global role."