The European Commission has taken the first step to challenge the British government over the legality of the multi-million pound grey market in the trading of take-off and landing slots at London Heathrow airport.
It has written to the UK requesting information about several transactions made in the last couple of years by airlines such as British Airways, Virgin Atlantic and Australia's Qantas.
A spokesman for the British government's transport department said it had received "a notice of infraction proceedings" from the European Commission relating to "certain slot exchanges" at Heathrow.
The letter suggests the UK could be breaching the 1993 European slot regulation through its failure to take action against Airport Co-ordination Ltd (ACL), the company that allocates slots at congested UK airports.
ACL has approved the various transactions that have taken place, including the sale earlier this year by FlyBE and Air France of six daily slot pairs to Qantas and Virgin Atlantic for about £40 million (€60.5 million).
It is understood that Qantas paid close to £20 million for two pairs of slots, setting a new record for prime daily slots at Heathrow, the world's busiest international airport.
The market rate in previous deals had valued a prime slot pair at between £5 million and £6 million.
The chief buyer has been British Airways, the leading operator at Heathrow, which controls about 41 per cent of all take-off and landing slots at the airport.
In a series of transactions during the last two years BA has acquired slots from United Airlines, Swiss International Air Lines and SN Brussels Airlines as well as from a number of smaller carriers.
BA acquired two pairs of slots from United for around £12 million and eight of Swiss's 14 Heathrow daily slot pairs in a transaction understood to be valued at about £35 million.
Heathrow is one of the world's most congested airports and there are virtually no free slots during most of its operating hours.
Under European rules there is no open trading of slots, and prime slots only change hands through a grey market, in which slots ostensibly are "exchanged" (prime slots for unattractive slots) in return for a normally undisclosed payment.
Britain's transport department said: "The UK believes these exchanges are in accordance with the rules governing slot allocation. We are drafting a response defending the UK's position."
The UK has two months to respond to the Commission, which will then have to decide whether it wishes to proceed with the issue. Ultimately Brussels could take legal action against the UK in the European Court of Justice.
