Apax Partners LLP, the London-based private-equity firm, raised 5.8 billion euros ($7.5 billion) for its eighth fund, more than 3 billion euros less than its original target.

The Apax VIII fund increased the company’s total committed capital to more than $40 billion, Apax said today in a statement. It began the fund in May 2011 with a 9-billion-euro target, holding an initial close, the point at which the manager can begin to invest, of 4.3 billion euros in March 2012, said Sarah Rajani, an Apax spokeswoman.

Private-equity funds are having more difficulty raising capital, with the average time spent marketing rising to 18 months from 14 1/2 months in 2008, according to data provider Preqin Ltd.

“We believe the strength of commitments received reflects confidence in our ability to add real value and is a clear endorsement of Apax’s global growth investing model,” Martin Halusa, Apax’s chief executive officer, said in the statement.

Apax said about 51 percent of the commitments were in euros and 49 percent were in U.S. dollars, with North American investors making up the largest portion.