Ashurst remains Apax's primary adviser and will continue to get the bulk of its work, but Freshfields Bruckhaus Deringer, its secondary adviser, will have to share the remainder with Weil.
The Lawyer can also reveal that Apax's panel will run for just four more months, until June, when the three advisers will face a further review.
The panel firms agreed to fee discounts of more than 10 per cent on hourly rates, although Apax will pay something out on aborted deals.
It is understood that when Freshfields signed up last June, Apax reserved the right to appoint another law firm as a secondary adviser.
Freshfields finance partner David Eirera has fantastic contacts at Apax, but during the panel review process Munich partner Peter Nussbaum, who was close to Apax chief executive Martin Halusa, quit the firm for Milbank Tweed Hadley & McCloy.
The appointment is a coup for Weil's London office, which was hit last year when major client Hicks Muse Tate & Furst went through an acrimonious split between its US and European operations.
London managing partner Mike Francies advised the US side, giving rise to serious concerns that Weil might be frozen out of future European instructions.
Weil's Mark Soundy, who worked closely with Apax before quitting his former firm Travers Smith Braithwaite last summer, was a key factor in the Apax win. Travers and Clifford Chance, both long-time advisers of Apax, were disappointed to have missed out on a panel place last year.
Ashurst, Freshfields and Weil declined to comment. Head of financial services at Apax Stephen Green, who ran the review, was unavailable for comment.