Allowing Heathrow to expand will create “a serious obstacle” to meeting the UK’s commitments on climate change and reducing air pollution, a leading scientist has warned.

Environmental groups expressed dismay at the Government’s decision to give a third runway at the airport the go-ahead – and Greenpeace vowed to challenge it in the courts.

Activists also signalled they would launch a campaign of direct action by locking themselves together on a mock runway outside the Westminster Parliament.

The Government said it believed a new runway at Heathrow could be created while still meeting the UK’s “obligations” to cut carbon emissions.

But an analysis of official figures by the Carbon Brief website found that the rising demand for flights could mean aviation could emit up to two-thirds of the maximum amount of greenhouse gases that the whole of the UK can produce by 2050 if it is to stick to its commitments on climate change.

This would mean drastic cuts in emissions produced by other sources, particularly from power stations, transport and heating systems. One expert who backed the decision to expand Heathrow rather than Gatwick, said other sectors of the economy would have to reduce greenhouse gases by more than 80 per cent by 2050 to comply with the UK Climate Change Act.

But Professor Joanna Haigh, co-director of the Grantham Institute for Climate Change and the Environment, said she thought the Government had made a mistake.

“Expansion at Heathrow, or any other airport, will create a serious obstacle to the UK meeting its greenhouse gas and air quality targets,” she said.

“Increased flights, more traffic on roads in and around the airports, and emissions from the new construction will add to an already woeful situation, particularly in London and the South East of England.

Protest against Heathrow and Gatwick expansion staged outside parliament

“While I appreciate that current business projections suggest the need for more flights, by the time new runways are operating, they could look seriously outdated.

“Rolling out of the best quality telecommunications networks to all businesses, including fast and reliable internet on trains, could benefit business productivity whilst reducing pollution by greenhouse gases and particles.”

She said the Government should have supported research into green alternatives to existing transport systems.

“Instead of expanding airports, I feel it would be better to redirect funds to accelerate research and development in clean transport and telecommunications and to prioritise the UK’s responsibilities to air quality and environmental health,” Professor Haigh said.

Dr Marc Stettler, of the Department of Civil and Environmental Engineering at Imperial College London, said Heathrow was in a better location to serve the “growing demand for air travel” than Gatwick.

“It has better transport links to and from the rest of the UK and it currently offers a larger variety of long-haul destinations that will be important in a post-Brexit Britain,” he said.

If the number of flights was going to increase, Dr Stettler said cuts in greenhouse gases would have to be made elsewhere.

“In terms of climate change, growth in aviation will mean that other sectors of our economy will have to reduce greenhouse gas emissions by more than 80 per cent by 2050 to meet the Climate Change Act,” he said.

“This reiterates the need for an integrated national transport strategy that balances a growth in aviation with reductions in emissions from other forms of transport.”

Greenpeace launched an appeal for funds to help pay for a judicial review of the decision.

John Sauven, Greenpeace UK's executive director, said: “A third runway at Heathrow would be a waste of time, money and lives.

“It would make Londoners’ air more dangerous to breathe, contributing to an air pollution crisis that already kills thousands. And it would load the atmosphere with as much extra carbon as some entire countries pump out.

“We stand ready to take the May government to court on this, side by side with Conservative west London councils. This runway has been defeated before and can be defeated again.”

In its statement about the decision, the Government said: “Today’s announcement follows an unprecedented UN global agreement achieved earlier this month to combat aviation emissions.

“Under the deal, airlines will offset their emissions with reductions from other sectors to deliver carbon neutral growth for the aviation sector from 2020. The Government believes that a new runway at Heathrow can be delivered within the UK’s carbon obligations.”

However Dr Doug Parr, Greenpeace UK’s chief scientist, tweeted: “Government believes new runway at Heathrow can be delivered within UK carbon obligations. Nobody who understands thinks so.”

In pictures: 70 years of Heathrow Show all 22 1 /22 In pictures: 70 years of Heathrow In pictures: 70 years of Heathrow Inside one of the terminal tents in 1946 The year the airport opened. Comfortable armchairs and flowers try to distract from the conditions Graham Bridges collection In pictures: 70 years of Heathrow An aerial view of the airport in 1949 Construction of the runway layout and Central Area are under way In pictures: 70 years of Heathrow A Pan Am crew checks out the Boeing Stratocruiser N1029V Clipper Golden Eagle in 1954 During the early 1950s, Pan Am and American Overseas Airlines operated Statocruisers into London Airport in direct competition on the North Atlantic route operated by BOAC In pictures: 70 years of Heathrow One of the first official London Airport guidebooks C.1953, priced 1s In pictures: 70 years of Heathrow In 1950 a permanent concrete terminal building was built This replaced the tents previously used at London Airport North and is seen still in use for charter and cargo flights in this 1959 view via Graham Bridges In pictures: 70 years of Heathrow BOAC check-in desk in 1954 Inside the new London Airport North terminal building, just before the move to the Central Area Graham Bridges collection In pictures: 70 years of Heathrow Air traffic control tower in the 1960s Inside the visual control room CAA Archives via Pete Bish In pictures: 70 years of Heathrow Rear cover of the 1956 guidebook Showing a plan of the airport at the time, with entrance prices to the spectators’ viewing terraces and for airport coach tours In pictures: 70 years of Heathrow Spectators in 1958 How close can you get? As soon as the Central Area was open, spectators were afforded unprecedented views of the airliners In pictures: 70 years of Heathrow Terminal 3 was opened as the Oceanic Terminal on 13 November 1961 It was built to handle flight departures for long-haul routes. Renamed Terminal 3 in 1968, it was expanded in 1970 with the addition of an arrivals building In pictures: 70 years of Heathrow Inside Terminal 3 in 1969 Check-in desks for BOAC and QANTAS airlines In pictures: 70 years of Heathrow Plane spotting on Heathrow’s viewing terraces in the 1960s Wrap up warm, take your spotting logbooks, pen and binoculars and get your mum to pack your sandwiches In pictures: 70 years of Heathrow No 1 Passenger Building Also called the Europa Building. In this photo, taken on 22 June 1963, flags of the many airlines it serves are flown Lee Holden In pictures: 70 years of Heathrow Luggage-trailer-towing Routemaster buses When BEA and BOAC merged to form BA on 1 April 1974, both fleets had to be repainted in the new livery, but so did all the ground support equipment In pictures: 70 years of Heathrow The entrance to the traffic tunnel in 1974 A Lufthansa Boeing 737 is seen on the runway In pictures: 70 years of Heathrow A 40 per cent scale model of Concorde In September 1990 it was erected on the roundabout at the entrance to the tunnel that passes under the northern runway at Heathrow Airport. It was built in four main parts, with an 80ft-long central fuselage section, to which the wings and tail fin were attached. The completed model was placed on the roundabout in September 1990 and was monitored by CCTV and surrounded by an infrared perimeter alarm that was connected to the local Heathrow police station to ensure it was not vandalised In pictures: 70 years of Heathrow Heathrow Airport's 50th anniversary On 2 June 1996, Heathrow marked its anniversary with a flypast of representative airliner types that have served the airport over the years. This culminated in a formation flypast by Concorde with Hawks of the RAF Red Arrows aerobatic team In pictures: 70 years of Heathrow The roof of Terminal 3’s car park One of the last bastions for plane spotters and spectators was here. This is the unfriendly notice that greets anyone who attempts this today Richard Vandervord In pictures: 70 years of Heathrow On 24 October 2003 BA withdrew its Concordes from service The final scheduled commercial flight was BA002 from JFK operated by G-BOAG. Here we see three of the Concordes parked together outside the BA hangar on 8 November 2003 following withdrawal John Hughes In pictures: 70 years of Heathrow The new control tower Costing £50 million to construct, it gives controllers an excellent 360-degree panoramic view NATS photograph In pictures: 70 years of Heathrow The new Terminal 2 The Queen’s Terminal In pictures: 70 years of Heathrow Looking due west down Runway 27L

Activist groups vowed to continue the fight against the third runway.

Stephanie Nicholls, of Reclaim the Power, said: “We can honour our commitments to tackle climate change, or we can build new runways – we can’t do both.

“Aviation expansion anywhere is irresponsible, and globally will impact the most on the people who’ve done least to cause the problem. Climate change is already hitting poorer communities in the global south, who are the least likely to ever set foot on a plane.

“When the Government won’t follow its own rules, it’s time for normal people to step up and take action.

“Following today’s announcement climate activists, council leaders and local residents will be standing together to make any new runways undeliverable. If the Government thinks they can override local opinion, climate science and their own commitments, they’ve got another thing coming.”

Shona Kealey, a spokesperson for Plane Stupid, said: “Two weeks ago, enough countries agreed to ratify the Paris Agreement for it to come into force. Last week, the Government’s climate advisers issued a report saying reducing aviation emissions should be a priority if we’re going to honour the Climate Change Act.