Skadden Arps Slate Meagher & Flom and Simpson Thacher & Bartlett have won lead roles for Apax Partners as the buyout house closes in on a multibillion-pound deal that could become one of the largest European leveraged buyouts since the start of the credit crisis.

Apax is in exclusive talks to buy Denmark-based facilities giant ISS after putting in an $8.5bn (£5.5bn) bid for the company.

Skadden is advising Apax on corporate aspects of the bid, with the firm's Germany M&A head Matthias Jaletzke leading. Meanwhile, the London office of Simpson Thacher is advising Apax in relation to financing, with finance partner Ian Barratt at the helm.

ISS is also considering launching an initial public offering (IPO) if the sale does not go ahead, with owners Goldman Capital Partners and EQT – owned by the Swedish Wallenberg business family venture – launching a dual-track process to sell or list the business earlier this year.

Danish firm Gorrissen Federspiel is understood to be advising ISS and its owners in relation to both the sale and IPO proceedings, while Danish rival Plesner is understood to be advising banks that would be involved in the potential IPO.

Apax's offer for ISS trumps those from rival consortia bids. One consortium reportedly includes the likes of Blackstone and Bain Capital while the other reportedly includes CVC Capital Partners and Apollo.

Commenting on the deal, one City partner said: "This deal will certainly be the biggest deal since the Alliance Boots deal both in deal size and debt size, should it go ahead."

Alliance Boots was taken over by private equity giant Kohlberg Kravis Roberts & Co for £11bn in 2007.