The quality of service received by passengers and airlines at Heathrow airport will come under the scrutiny of the UK's competition regulator, as part of a wide-ranging investigation into the country's largest airports operator.

The Competition Commission said its investigation into BAA, which also owns Gatwick and Stansted airports, will also consider whether the company's ownership of seven UK airports impacts the amount of money it is willing to invest in improving facilities.

Details on the investigation come amid mounting criticism of Heathrow, the country's busiest airport. London's main hub has come in for intense criticism for delays and poor service, intensified by rising security requirements and the summer rush to get abroad.

Kitty Ussher, the City minister, recently said that "Heathrow hassle" could threaten London's position as a financial centre.

The Office of Fair Trading prompted the Competition Commission review in March when it concluded that BAA's stranglehold on the region could "prevent, restrict or distort" competition.

Airlines, including British Airways and Ryanair, have been vocal in their opposition to BAA's joint control of the three largest airports in the South East of England, arguing that the absence of competition is failing the industry.

British Airways argues that owning Heathrow and Stansted airports BAA is holding back the development of new runways, and in doing so holding back much needed expansion in air capacity in the south east of England.

Willie Walsh, the chief executive of British Airways, said today: “Common ownership is the root cause of the failure to expand Heathrow’s runway capacity," adding that a more competitive airports market around London would encourage the development of new facilities.

Ryanair complains that BAA's charges are too high, made possible by the lack of competition. It also complains that the airports group is trying to push through expensive plans for expansion at Stansted where the same capacity increase could be increased at lower cost.

BAA argues that joined-up ownership puts it in a better place to deliver more capacity, allowing it to use its balance sheet to invest more efficiently.

The Competition Commission said its investigation into service levels would extend to terminals, runways, ancillary facilities, and other facilities including security. It said "common ownership could affect the incentives as to the quantity and level of expenditure".

The regulator will also question whether Southampton airport, another BAA division, should be considered part of the South East of England airports market, as one airline suggested it should.

"A likely starting point for our analysis is an assessment of the extent of market power held by each BAA airport," the regulator said.

Stephen Nelson, the chief executive of BAA, said today: "What London airports do not need are structural changes that will seriously delay the delivery of the investment that is urgently needed to improve the passenger experience and increase capacity."

He said that BAA has "the will and the ability to make the investment London airports need". However he said that the group in turn required political support for its plans.

Industry experts have long speculated that BAA could be forced to sell one or more of its airports by the Commission. It is thought a sale of Gatwick Airport would be most likely.

Heathrow Airport's Terminal 5, which will host most of British Airways' flights into and out of the airport, opens in March next year. A new terminal, Heathrow East, is currently in the planning stage.

The scope of the investigation will also cover airport services in lowland Scotland.

The investigation comes after a Grupo Ferrovial-led consortium last year took BAA private in a £10 billion deal.

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