A researcher at the University of Bath says a third runway at Heathrow airport will cause long-term environmental damage.

Transport Secretary Geoff Hoon yesterday controversially agreed to the £9 billion scheme, which would be built between 2015 and 2020.

But Dr Marcelle McManus, an environmental scientist in the university's Department of Mechanical Engineering said: "The extra runway at Heathrow will not solve any transport or economic problems and will reduce our ability to meet our climate change targets.

"A reduction of global warming gas emissions is vital if we are to stem the potential impact of climate change. Heathrow currently generates 50 per cent of the UK aviation emissions. An expansion of this, when we have tight carbon reduction targets to meet, is extremely unwise.

"The Government argues that the third runway is necessary in terms of the economy. It also argues that CO2 emissions will not be reduced if the runway isn't built as the flights and airlines will simply move to larger capacity airports in mainland Europe.

"However, this is ill-considered as many of the flights from Heathrow are short haul."

She said that if the money were spent on improving other forms of high speed transport, such as investment in better trains, the need for such flights "could simply disappear" and there would be no need for the expansion.

She added: "The Government has stated that there will be an initial limit on the use of the new runway and that there will be mechanisms put in place to ensure that only newer more efficient planes can use it. However, the increase in traffic will outweigh any potential improvement in technology.

"There is also evidence that some of the new technology proposed by airlines will increase noise levels beyond legal limits, hence making their proposals unviable.

"It is unlikely that there will be any long term gain to this decision and there will definitely be long-term environmental damage as a result."

Mr Hoon's package of transport announcements yesterday also included plans to open up the hard shoulders or add extra lanes to a number of motorways.

These £6 billion proposals would affect the M4 between the M32 and Almondsbury and the stretch of the M5 between Almondsbury and Cribbs Causeway.

Work is due to start by March 2010.

He said: "We have successfully and safely opened up the motorway hard shoulder on the M42 at busy times, giving us confidence that we can do the same elsewhere on the network. This will deliver more reliable journey times and add a third more capacity – at a lower cost than a more conventional road widening scheme, and with fewer environmental impacts.

"That is why I am announcing that this innovative technique will first be extended to the most congested sections of England's motorways and in the longer term will be rolled out across the core motorway network linking our major cities.

"The investment package of up to £6 billion that I have set out today will also fund more conventional widening projects as well as junction improvements and other innovative projects to tackle congestion. In the south west, drivers will benefit from safe, faster and more reliable journeys as extra road capacity is added where it is most needed."

He is also considering electrifying the main rail line between London and Bristol.