IT WAS one of the biggest, and most heavily leaked, trust-busting decisions in British corporate history. On March 19th the Competition Commission confirmed its provisional decision that BAA, the firm that monopolises Britain's big airports, should be dismembered, selling off two airports near London as well as Edinburgh or Glasgow. Aware that the watchdog was planning such a move, BAA (which thinks the decision “flawed” and is pondering an appeal) had already put Gatwick airport (Britain's second biggest) on the market.

This week's news will come as a relief to passengers familiar with the ordeal of Heathrow, the world's busiest international airport—the delays and crowding that result when an airport designed to handle 45m people a year crams 67m through its doors. Heathrow's runways operate at virtually full capacity, so the smallest hitch causes a cascade of delays. The commission acknowledges that policy-making has been poor and the regulatory regime inadequate. But it lays much of the blame on BAA's near-monopoly on air travel in Britain. Ideally, an expanded Gatwick or, to a lesser extent, Stansted could relieve the pressure. But crowded Heathrow generates plenty of profit and Gatwick and Stansted are also owned by BAA, so reducing congestion is not the firm's top priority. Splitting ownership of the airports should encourage competition between them.

There will be less cheer at Ferrovial, the Spanish construction firm that bought BAA in a swashbuckling £10 billion debt-financed deal in 2006. The forced sale of assets offers the thin consolation of providing cash to pay down some of Ferrovial's £20 billion debt. But Gatwick and Stansted are going on the market at a time when prospective buyers will find it hard to get cash from credit-crunched markets. And aviation is in dire straits. In December Giovanni Bisignani, head of the International Air Transport Association, said his industry was facing “the toughest revenue environment in 50 years”. On March 16th the Civil Aviation Authority (CAA), which regulates BAA, said passenger numbers at British airports had fallen by 1.9% in 2008, only the fourth decline since 1945. The fall was steepest in the final quarter of the year, and the CAA reckons that numbers will continue to drop throughout 2009.

All this means that BAA may struggle to get a good price (indeed, on March 13th it extended the bidding deadline for Gatwick by a month, ostensibly to allow more time for due diligence). Heathrow's size and its status as an international hub make it a formidable competitor. Gatwick's regulated assets alone are worth around £1.7 billion, according to the CAA, but many analysts doubt whether the final price will be much more than £1.8 billion (expansion there is forbidden before 2019).

Stansted will be even cheaper, says David Starkie, an aviation expert, who points out that its growth was fuelled for years by a cross-subsidy from Heathrow that was abolished in 2003. The government plans a big expansion, but locals are fiercely opposed and demand may not justify more capacity. Passenger numbers at Stansted fell by 6% last year, whereas numbers at Luton—a rival airport that is not owned by BAA—rose by 2.6%.

The Competition Commission takes a rosier view of future demand, and thinks that breaking up BAA is the best way to encourage new capacity to meet it. Last year ministers approved the building of a third runway at Heathrow (although few believe it will happen). But increasing concern about climate change may alter things. Air travel's share of planet-heating carbon emissions is expected to rise. And the European Union plans to include aviation in its emissions-trading scheme, which should raise ticket prices. More competition will encourage BAA to do better—but it will take a brave businessman to try his hand at it.