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.