ATPI, which employs more than 1,400 staff across 52 offices in five continents, specialises in the marine, oil and gas and construction sectors.
Equistone invested in ATPI in a deal worth £73m in 2008, since which time the business has continued to grow.
In recent years, the company has formed new partnerships in Vietnam, Indonesia, Spain and Portugal, and opened two new offices in Australia.
CEO Graham Ramsey will continue to lead the business following the transaction.
The sale of ATPI follows a number of other transactions by Equistone in the travel sector, including most recently the €1bn (£791m) sale of international travel-related payment services firm Global Blue, the acquisition of bespoke travel company Audley Travel and the €200m (£162m) sale of travel agency Karavel-Promovacances.
John Walker, director at Equistone Partners Europe, said: "It has been a privilege to work with ATPI over the last four years, helping them to develop the business into one of the fastest growing brands in corporate travel, despite the difficult trading conditions in the travel sector during this time. This exit has generated a good return for our investors in Fund III and leaves the Company in great shape for future growth with ICG’s backing. We wish Graham and the team the very best."
Ramsey added: "I believe that there are significant opportunities ahead in the travel sector and look forward to working with ICG to explore these."