The Competition Commission is due to set out the main issues it will study in its investigation into airport operator BAA.
The Competition Commission (CC) says its study into the market domination of airport operator BAA will focus on the firm's level of customer service.
BAA, - which owns Heathrow, Gatwick and Stansted and four other UK airports - was referred to the Commission by the Office of Fair Trading.
While the CC said it was aware BAA has faced extensive criticisms, especially over Heathrow, Gatwick and Stansted, it maintained an open mind at this stage.
Amid airline calls for BAA to be broken up, the OFT said it suspected BAA's high market share damaged competition.
The firm was referred to the CC by the Office of Fair Trading in March, amid airline calls for BAA to be broken up.
BAA insists the problem is lack of capacity, not how many airports it has.
BAA says lack of capacity is the issue, not its ownership of seven UK airports.
The firm, which was bought by Spanish group Ferrovial in 2006, said in March that it had no intention of selling any of its UK properties, which also include Southampton, Glasgow, Edinburgh and Aberdeen.
"At this stage, we have no preconceived ideas of what our conclusions might be; and if we were to identify competition problems, what the appropriate remedies might be," the commission said in a statement.
Political criticism
However, criticism of BAA has increased in recent weeks, with attacks from both Labour and the Conservatives.
BAA PASSENGER NUMBERS Heathrow - 67.7 million a year Gatwick - 32 millionStansted - 22 millionSouthampton - 1.5 million Glasgow - 8.7 millionEdinburgh - 8 millionAberdeen - 2.7 million Source: BAA
London Mayor Ken Livingstone has said that Heathrow "shames London", while the Conservatives have described BAA as "an airport operator who seems more concerned with running shopping malls than an efficient [Heathrow] airport".
BAA has also been criticised by some quarters for seeking and gaining a legal injunction to ban some green activists from protesting at Heathrow about climate change.
The Commission must make its final report by March 2009, but is expected to publish its preliminary findings in summer 2008, with the full report following by the end of that year.
It is also looking at the level of charges that BAA is allowed to impose on airlines for using its airports, with the report on this issue due by the end of September this year.
BAA has a 60% market share of all UK passenger flights, rising to 90% in the south of England.
