Heathrow is one of the world's busiest and most frustrating airports. It is also one of the most popular — despite permanent congestion, stoppages during the summer peak and widespread anger over queues, intrusive security and poor passenger relations. In the past ten years the number of passengers using Heathrow has risen from 55.7 million to 67.3 million annually. But between 2002 and 2006 the average flight delay increased by more than two minutes to 18.8 minutes. Around 50 per cent of landing aircraft are held in a stacking pattern, as the airport's two runways are already operating at 98.5 per cent capacity (compared with 74 per cent at Charles de Gaulle airport in Paris). The expected announcement today by Ruth Kelly, the Transport Secretary, that the Government is to put the case for a third, short runway is therefore not only welcome to millions of passengers; it is essential if Britain is to remain competitive in a global economy.

Her statement that a new runway would not breach strict noise and environment limits follows official consultation. It is an important finding, and certain to raise a storm of controversy, for it appears to sanction an expansion that will almost double the number of flights from 480,000 a year to 800,000. The main obstacle, until now, has been compliance with the mandatory EU air quality limits that will apply from 2010. Campaigners against expansion have insisted that the aircraft emissions, added to the additional traffic pollution generated on the ground, will make this impossible. As a result, all the talk in recent years has been of siting a new runway at Stansted, an assumption that has raised a storm of protest by those living in the region.

It has long been clear, however, that the Government has always, and rightly, regarded Heathrow as the priority. As Britain's premier airport, it is of far greater importance to the business community and to international passengers using London as a stopover or transit route. Overcrowding and delays, however, have already begun to erode Heathrow's competitiveness: the number of scheduled international destinations has fallen from 227 in 1990 to 180 in 2006; the number of domestic destinations served has fallen by 50 per cent, and Heathrow is falling behind Schiphol (Amsterdam), which has five runways, Charles de Gaulle with four and the ambitious Frankfurt.

Protesters will predictably question the Government's measurements, which appear to show compliance with noise and air quality limits. Some figures may indeed be based on optimistic projections of cleaner aircraft and car engines. But there must be no capitulation to the anti-Heathrow lobby that is not only trying to cut the benchmark for measuring noise levels from 57 to 50 decibels and place more curbs on night flights, but is wildly exaggerating the likely impact. Nor can the argument about climate change be harnessed to any anti-Heathrow campaign without also being applied to all airports across Europe.

Using the two runways for simultaneous landings, and building a third one, will create benefits estimated at more than £9 billion a year. More must be done to improve access, including direct rail connections. The onus is also on BAA to show that it can run the three main London airports efficiently if it is not to lose its monopoly. But there must be no wavering in pursuing expansion at Heathrow over Stansted or Gatwick. Britain's prosperity depends on it.