Coller Capital has raised $5.5 billion to buy private equity investments from banks and other investors, tapping into a growing niche that has seen some of the biggest pools of capital raised by buyout firms over the last year.

As traditional private equity groups have found new capital hard to raise and deals tough to find since the credit crisis, firms like Coller and rival AXA Private Equity have seen their market boom as private equity investors, including banks, pension funds and endowments, sell assets to free up capital.

Coller said it raised $5.5 billion on Thursday, 10 percent more than it initially targeted for its sixth so-called secondaries fund.

"We have already committed $800 million of the new fund's capital, and we have a very full pipeline of potential investments," said chief investment officer and founder Jeremy Coller.

Coller was one of the first to specialise in buying positions in private equity funds from investors, as well as acquiring portfolios of stakes in private companies from their original backers.

More recently banks, selling off their private equity interests in preparation for more stringent regulation, have provided a rich seam of investment, and have helped a niche part of the private equity industry grow.

AXA Private Equity, the private equity arm of the French insurer, raised $7.1 billion for a new fund earlier this year, more than double its initial target, and U.S. group Lexington raised $7 billion last year.

AXA private equity expects between $40 billion and $50 billion of banks' private equity holdings to come up for sale over the next couple of years, as financial institutions sell off a wide range of holdings to free up capital.

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