Private equity firms Bain Capital and Hellman & Friedman are negotiating as a team to acquire Lehman Brothers investment management unit, according to people familiar with the discussions as reported by Bloomberg.

The pair have reportedly bid about $4 billion for the unit, which includes the Neuberger Berman fund management business.

Lehman plans to sell this business - which oversees $273 billion in assets including private equity funds - and a brokerage business serving wealthy clients. The firm is selling its North American investment banking business to Barclays.

The investment unit made $361 million on $2.3 billion of revenue in this year to August, according to a contemporary Sanford Bernstein research note valuing the division at $7 billion.

The US bankruptcy court in Manhattan must approve the investment management business sale. It has already approved the disposal to Barclays, so that deal could close next week unless a higher offer is received.