Slaughter and May and Herbert Smith have won lead roles on one of this year's largest M&A deals – the £5.2bn purchase of Danish facilities outsourcing firm ISS by UK security group G4S.

The acquisition is set to make the Crawley-based security services provider the world's largest combined security and facilities service provider, with revenues of around £15.9bn.

Herbert Smith advised G4S with a team led by the firm's M&A co-head Stephen Wilkinson, corporate partner Chris Haynes and US securities partner Stephen Thierbach.

Meanwhile, Slaughters advised ISS's sole shareholder FS Invest II as well as its owners, Goldman Sachs Capital Partners and EQT Partners.

The Slaughters team was led by private equity head Jeff Twentyman and senior partner Chris Saul on the M&A side, alongside financing partner Robert Byk.

Norton Rose also acted on the deal, advising G4S on competition issues with a team led by Mark Tricker.

Bruun & Hjejle took the lead Danish law advisory role for G4S, fielding a team led by partners Henning Aasmul-Olsen, Jeppe Skadhauge and Mogens Ebeling. Gorrissen Federspiel advised the sellers on local law led by partner Tobias Linde.

The deal will see G4S pay £1.52bn for ISS, with half in cash and half in shares, while also assuming £3.67bn of debt.

The deal comes after ISS abandoned a planned $2.8bn (£1.8bn) initial public offering (IPO) earlier this year amid market volatility, after facing an $8.5bn (£5.5bn) takeover bid from Apax Partners at the end of last year.

That aborted deal saw Skadden Arps Slate Meagher & Flom and Simpson Thacher & Bartlett in lead roles for Apax Partners, while Gorrissen Federspiel advised on the abandoned IPO.