No one would argue against the desirability of extra leg room on flights. Even the airlines, if they did not fear it would hit their bottom lines, would love to give economy passengers enough space to stretch out.

A head of steam has built up in favour of change: a long-haul package holiday customer has been awarded compensation for discomfort by the small claims court, and a group of deep vein thrombosis victims has won the first stage of their planned legal action against airlines.

But how can legroom be increased? And would consumers be happy to put their money where their mouths are?

Because of the way aircraft are divided into different sections by galleys, bulkheads and cabin dividers, reducing the number of seat rows is trickier than it seems. To provide passengers with two extra inches of seat pitch - the distance between the back of your seat and the nearest point of the one in front - it would typically be necessary to remove at least four rows from a short-haul Boeing 757-200 or a long-haul 767-300. That works out at a loss of 24 or 32 passengers respectively. And because most seats are in the middle section, more legroom would be gained there than at the front or rear of the aircraft.

As for the cost involved, it is difficult to assess the impact on scheduled airlines because they offer so many fares and classes. British Airways, for example, has four different cabins on long-haul flights, with the new World Traveller Plus somewhere between economy and business. Business travellers already pay more than double the economy fare for less than twice the legroom (although their seats are wider, too). It might be possible to further nudge up premium fares - but airlines will be anxious not to provide so much comfort in economy that their top fare payers migrate there.

If greater legroom persuaded more people to fly at higher fares during periods when average load factors (the percentage of seats filled) are low, carriers could even benefit. American Airlines, which has increased seat pitch to between 34 and 36 inches on US domestic and international routes, claims it has lured customers from its rivals.

Charter airlines have less room for manoeuvre. Their business is already based on average load factors of 95% or more, and removing seats would increase the price of package holidays, hitting travellers on tight budgets. Experience suggests the burden would also fall on people travelling in the school holidays, when airlines rely on jets flying full.

However, it is easier to estimate the likely impact on charter than on scheduled passengers, because tour operators pay a set rate for seats, typically thought to hover around the £200 mark for a round trip between Gatwick and Orlando or £100 for a flight to southern Spain.

One airline source estimates that to increase seat pitch to 32in on long-haul charters would add £30-35 to the average cost of a seat, while moving from 28in to 31in on short-haul flights would add about £15. My own crude calculations suggest those figures might be slightly exaggerated; it is hardly in the industry's interests to underestimate the impact.

The major UK charter carriers already offer greater legroom in wider seats on longer flights - offered by tour operators at a premium. (Airtours charges an additional £149 for an average of five extra inches.) But when Britannia Airways introduced a third option - a standard width seat with a 34in pitch - at a lower price supplement, it failed to attract enough customers.

Scheduled operators British Airways and Virgin Atlantic have also introduced sections between business and economy, both offering a generous 38in. Not so long ago, that was the sort of legroom in business class. But in the battle to woo high fare payers, airlines are now offering business cabin pitches of 47in and more and are increasingly installing sleeper seats. In the current struggle to sell premium fare seats, BA has been offering economy passengers the chance to upgrade to World Traveller Plus on some routes for £150 one way. At least one tour operator believes that if this sort of offer were made permanent, it would attract a lot of business. The cheapest current return to New York in the more spacious cabin is £678.80 (normally £733.80). The lowest current economy fare is £258.80.

These comparisons show that, as with business class, the higher prices for seats in charter premium and economy-plus cabins bears little relation to the extra space provided. Given Britannia's experience, it could be concluded that while there is strong demand for extra legroom from a limited number of people prepared to fork out a handsome supplement, it is questionable that enough demand exists for, say, two extra inches at a more modest premium.

Eventually, airlines may find their hands forced for safety reasons. A study commissioned by the Civil Aviation Authority (CAA) has recommended an increase from 26in to at least 28.2in as the minimum permitted seat pitch and has suggested that, ideally, that figure should rise to 29.4in. The motive for this was the belief that people, on average, are getting taller and they need to be able to evacuate safely in emergencies. But Britain is thought to be the only country to impose such a rule and the CAA believes more people would benefit if a minimum measurement were agreed first by Europe's Joint Airworthiness Authorities, of which it is a member.

Neither would airlines already offering at least 28 inches escape change: they tend to measure pitch between the same points on seat legs, while the CAA measures from the front of the seat back cushion to the nearest point on the seat in front. The thickness of the cushion reduces the measurement by around two inches.

According to Ian Hamer, chairman of the watchdog Air Transport Users Council, "We want all airlines to inform passengers when they book what their seat pitches are - though they have raised concern about what happened if they have to bring in a different aircraft with less space at short notice. We also have to persuade passengers to ask for it."

Passengers, he says, "want the most comfort they can afford". But even if they are willing to pay for it, they cannot buy what is not available. Perhaps it is simply that airlines are nervous about going out on a limb. "Nobody," says Hamer, "wants to take the plunge."

Who offers what?

Here are the replies of airlines when asked for their average seat pitches. Comparisons are not made easy by the apparent confusion among some carriers about their own legroom. The space allocated to a seat varies on most jets because of the cabin layout. Measurements given are in inches.

Long haul

British Airways: 31

Air Canada: 33-34

Qantas: 32

Virgin Atlantic: 31

United: 32 (37 for frequent passengers cashing in mileage points; sometimes these are available on a first come, first served basis)

Britannia: 30 (Europe 28) Premium 35

Air 2000: 30 (premium 35)

Airtours: 30 (premium 35)

JMC Airlines: 30 (premium 34)

Monarch: 30 (premium 34)

American: 34

Singapore Airlines: 32

SAA: 31 (some seats at 34 on older 747s or on a first come, first served basis).

Lufthansa: 34

Short haul

BA: 31

Major charter airlines: 28-29

easyJet: 29 or 30, odd ones 32

Go: 29 (some seats 32)

Ryanair: 30-31