"The devil will be in the detail of this change in duty, but on the data available we see this as more positive for easyJet than for BA (British Airways)," said equity analyst Peter Caldwell in a Barclays Wealth research note.

"EasyJet tends to operate shorter routes, has high capacity utilisation and a very young, hence relatively fuel-efficient, fleet. BA has more of a focus on long-haul and, while it is starting to replace aircraft, it has a mature fleet."

The government said on Tuesday it would do away with its air passenger duty (APD) from November 2009 and replace it with new taxes on airlines based on each plane they fly.

How this is to be calculated will be the subject of a year of consultations between the government and airlines, green groups, passenger organisations and the wider business community.

"We don't know the mechanics of how the new tax will operate," said Neal Weston, head of policy and communications at the British Air Transport Association. "Will it be the age of the aircraft, the engines, the emissions, where it travels? We don't know the methodology."

Basing it partly on a plane's maximum take-off weight -- a standard already used for levying air traffic control charges -- is one possible factor, analysts said.

Plans to replace the APD come less than a year after an unpopular doubling of the duty by Prime Minister Gordon Brown, who was then Chancellor of the Exchequer.

The Treasury said the changes are aimed at raising an additional 520 million pounds from the aviation industry from the financial year beginning in April 2010.

APD annual revenues are on track to rise to about 2.3 billion pounds by then, meaning the government's new approach hopes to capture about 2.8 billion pounds per year from aviation, almost triple the intake in 2006.

ALL-BUSINESS

One loophole which the government said it will close starting in November 2008 affects all-business flights offered by airlines such as MAXjet, Eos and Silverjet.

Passengers on such flights will have to pay the higher of two APD rates, meaning it will double to 80 pounds per flight.

These airlines also fly older planes which could be subject to stiffer taxes when the APD is replaced in 2009.

"It'll be unhelpful for MAXjet, Silverjet and Eos, bearing in mind that they're at early stages of their development and some of them are not yet profitable," said analyst Wyn Ellis at Numis Securities.

A spokeswoman from Silverjet said the tax change could have a detrimental effect on the airline's environmental initiatives.

"It was a bolt out of the blue. There will be a substantial increase in costs. It will force us to review our environmental policy of offsetting our carbon emissions."

The Geneva-based International Air Transport Association (IATA), representing most of the world's airlines, urged Britain to look at policies rather than taxes to help the environment and consumers.

"Whether the tax is collected per plane or per passenger this sort of tax is ineffective in limiting aviation emissions," said an IATA spokesman.

He said airlines should be encouraged to invest in new technology while regulators should aim to make airports and flight routing as efficient as possible.

Other measures include emissions trading, the European Union's key tool to fight global warming.