ISS is one of the world's largest facilities services providers with a market presence in over 60 countries in Europe, Asia, South America, North America, and Australia. It is the world's fourth largest private sector employer having more than half a million people on its payroll.

It provides a wide range of services from cleaning and maintenance, transport and terminals, food hygiene, healthcare, catering and canteen management, front-desk receptionists to security guards and gardeners among others.

Post merger, G4S will be the second-largest non-governmental employer in the world, with more than 1.15 million employees, after US retail giant Wal-Mart.

Goldman and EQT, which had acquired Copenhagen-based ISS in 2005 for around $3.5 billion, had in August 2010 planned to either sell ISS or go in for an initial public offering.

In December 2010, London-based Apax Partners had entered exclusive talks after beating bids from others, including a consortium of private equity firms comprising Blackstone Group, Bain Capital, Nordic Capital and Clayton Dubilier & Rice, as well as from CVC Capital Partners working with Apollo Management. (See: Apax Partners in talks to buy Danish services firm ISS for $8.5 billion)

Apax was reported to have offered $8.5 billion (£5.4 billion) including debt, but a weak financing market made the bidders withdraw from the auction.

Crawley, UK-based G4S, is planning to raise £2 billion by selling new shares to help fund the acquisition.

G4S said that the combined company would be the world's largest security and facilities group, which had combined annual revenues of £15.9 billion last year or double the revenue of G4S, which generated £7.4 billion.

Under the deal, FS Invest II, the sole shareholder of ISS, will receive about £1.5 billion, half of which will be in cash, with the other half in G4S shares. G4S will also assume DKK31.5 billion, or $5.87 billion debt of ISS.

Since the two companies operations overlap in 40 countries, G4S said the deal would bring in an estimated annual saving of £100 million by 2014.

Nick Buckles, G4S chief executive, said, "We believe this acquisition will transform our business, significantly accelerate the delivery of our solutions strategy and create substantial value for shareholders."

G4S, formerly Group 4 Securicor, has its origins in Copenhagen, Denmark, having first started operations in 1901 as a guarding business by its founder Marius Hogrefe.

G4S was formed through the 2004 merger between Securicor plc and Group 4 Falck A/S's security business, and moved its headquarters to the UK.

G4S provides guarding services at various airports including Heathrow Airport, Oslo Airport, Brussels Airport and Schiphol Airport as well as at various facilities belonging to the US, UK, Canada and European Governments.

It also operates custody facilities, including prisons and electronic security systems. In addition, it provides cash management and logistics services for the main British and European Banks.

G4S owns the Wackenhut Corporation, a leading provider of security services in North America. They are also involved in risk management activities, consultancy and support in areas where infrastructure is limited, including services for governments, international peace and security agencies and multi-national corporations. Services also include land-mine clearance, ordnance management and training services. G4S also provides revenue protection services for railway companies across the UK.

The deal will also allow G4S, which already operates in 125 countries, to expand in markets such as Spain, Italy and Switzerland, where ISS has a strong presence.