Landing slots at Heathrow are so valuable that companies can spend years and millions of pounds negotiating to get only one aircraft into the airport.

Heathrow, the world’s busiest international airport, is so constrained in the number of flights it can accommodate that an airline wanting to set up operations there can pay up to £30 million for a good pair of take-off and landing times. When Continental Airlines, the American carrier, wanted to start operations from Heathrow this year it had to spend more than £100 million on only four slots, and there are rumours that other carriers have paid much more.

As a result, airlines jealously hoard their slots and in some cases these landing rights have become more valuable than the companies themselves. However, there is a “use-it-or-lose-it” rule in operation at Heathrow, so an airline such as bmi, the former British Midland, must keep flying to hold on to its slots - even if its aircraft are virtually empty.

It is, therefore, better for a carrier such as bmi to lose £20,000 per flight than to give up a £30 million slot. For bmi this is particularly important as it is trying to keep its value up for a potential sale this year. British Airways, Virgin Atlantic and Lufthansa are all interested in buying bmi, and the biggest attraction is the airline’s 11 per cent of Heathrow slots - the second-largest holding behind BA.

However, some aviation analysts believe that there are no legal grounds for these carriers to own the slots, and advocate that they should belong to the State and be leased to the highest bidder. High prices for rented slots would encourage only profitable flights, which would almost certainly mean full flights.

BA has also operated uneconomical flights from Heathrow in the past but has moved to a more efficient and flexible use of its landing times to get around the use-it-or-lose-it rule. Rising fuel costs, however, could make it more likely that the airline will sporadically cancel unpopular flights. Regular travellers on shuttle routes such as Heathrow to Edinburgh or Manchester have long suspected that BA cancels flights if there are not enough people on board, but this is likely to become a more regular event.

Passengers may have to get used to being stranded by airlines that do not want to lose money by transporting them.

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