 Britain will on Thursday outline plans to expand London's Heathrow Airport to help cope with a boom in air travel, despite fierce opposition from environmental groups. Transport Secretary Ruth Kelly will propose building a third runway for short-haul flights at Europe's biggest airport by 2020, according to a report in the Times. In the meantime, she intends to allow up to 60,000 more flights each year on the existing two runways, the report said. Supporters of the planned expansion say it will pump billions of pounds into the economy and allow Heathrow to compete with other international airports. Critics, however, say the extra flights will contribute to global warming, increase pollution and blight the lives of millions of people under the flightpaths. The developments are being closely watched by governments and campaigners across Europe, including Frankfurt, Paris and Stuttgart, where airport expansions are planned. Kelly will publish a consultation paper on Thursday that will say the expansion will not breach the European Union's air pollution limits and the government's own noise limits, the Times said. "We need extra capacity in the southeast," Kelly told the newspaper. "But fundamentally we need a global hub airport." Scientists say air transport contributes to global warming, and the carbon dioxide gas and water vapour emitted by aircraft are four times more potent at high altitude than at sea level. The government says it is committed to tackling climate change and plans to set legally binding targets for cutting CO2 emissions -- but it also backs an expansion of air travel, which is set to double in the next 25 years. Airport operator BAA, part of Spain's Ferrovial said the expansion would bring huge economic benefits through tourism, job creation and businesses relocating to be near Heathrow. "It is a very considerable economic powerhouse," BAA Chief Executive Stephen Nelson told BBC radio. British Airways Chief Executive Willie Walsh said benefits could be worth more than 9 billion pounds each year. Green campaigners question that figure. John Stewart, chairman of anti-airport expansion group HACAN ClearSkies, told the BBC: "There's a mantra here that it's important for the economy. What has never been worked out is how those figures are arrived at." Liberal Democrat Shadow Transport Secretary Susan Kramer said the plans "make a mockery of any attempts to tackle climate change". "It is time for ministers to listen to the public and stop any further Heathrow expansion," she said.