Olswang has secured a major mandate after being instructed on the proposed flotation of New Star Asset Management – worth an estimated £600m – opposite magic circle firm Linklaters.

Olswang will take a lead role advising the private equity house on the AIM IPO, which will mark its biggest-ever deal for New Star. Details are still to be made public, but the company is expected to float next month.

Corporate partner Simon Morgan is leading a 14-strong team advising New Star alongside fellow partners Mark Joscelyne (tax) and Michael Deeks (employment) and assistants Sarah Bolton and Tony Waller.

The deal cements Olswang's five-year working relationship with New Star. Morgan has been a main adviser to the client, which has £15.2bn in assets under management, since it was set up by former Jupiter Asset Management fund manager John Duffield in 2000.

"We have a very good relationship with New Star and I thoroughly enjoy working with them," said Morgan. "We often work for entrepreneurial companies and New Star is very much an entrepreneurial company."

Morgan, who advised New Star on a series of fund acquisitions in 2003, said he expected Olswang to continue working with the company after it lists on AIM.

The deal is a significant one for Olswang's private equity client roster after it also bagged a major role for the Tchenguiz family on its part in the consortium that acquired Somerfield for £1.1bn this month.

Mid-tier firms' advisory roles are often at threat when they opt to float. In June, Berwin Leighton Paisner was pushed aside by Freshfields Bruckhaus Deringer for the lead advisory role on the £5bn IPO of long-term client PartyGaming.

UBS Investment Bank, New Star's nominated adviser, bookrunner and global coordinator, is using Linklaters for advice. The magic circle firm is also advising joint lead manager Citigroup Global Markets UK Equity.

Linklaters corporate partner Matthew Middleditch, assisted by associate Alastair Keith, is advising the banks.