Virgin Atlantic
Virgin has already declared an interest in buying Gatwick as part of a consortium. Media reports suggest that it has spoken to a number of co-bidders, including Middle East investment funds.
Virgin Chief Executive Steve Ridgway said the company "would relish the opportunity to bid for Gatwick as part of a consortium and inject our customer service expertise into any future running of the airport".
For the company, it may be a way for it to compete with BA. Virgin boss Richard Branson fears that BA's proposed merger with American Airlines would reduce competition.
Global Infrastructure Partners
GIP owns a 50% stake in London's City airport, purchased in 2006. GIP is a $5.64bn (£3.15bn) fund that was set up by Credit Suisse and General Electric, but which operates at arm's length from its founding companies.
GIP focuses on energy, water/waste and transport, listing air transport infrastructure among its areas of expertise.
Macquarie Group
Macquarie is an Australian company that provides banking, financial, advisory, investment and fund management services to governments, companies and individuals. It has assets under management of more than $A225bn (£100bn).
Its private equity investment fund, Macquarie Airports Group, owns 50% of Bristol airport and 24.125% of Birmingham International airport. Macquarie also has a stake in Rome's Fiumicino and Ciampiono airports, as well as Sydney (Kingsford Smith) airport.
Hochtief
Hochtief is a German company that provides international construction services. It is Germany's oldest and largest construction company and was extensively involved in reconstruction of the country after the second world war.
Its subsidiary, Hochtief AirPort, was founded in 1997 and owns stakes in six airports: Budapest, Athens, Düsseldorf, Hamburg, Sydney and Tirana. HTA is one of the world's biggest private and independent airport managers and investors with a portfolio worth 1.3bn euros (£1bn) at the end of last year.
