London has pledged to host the greenest Olympic Games ever staged, but it could take years before the promises start to flower.

The pledge was made seven years ago by then-prime minister Tony Blair when London won the right to host the 2012 Olympics.

Bigger, more beautiful, cleaner: all Olympic Games make the same promises. But what becomes of the concrete infrastructure, the miles of new roads and the tonnes of waste?

"It's a slight oxymoron, the idea of the Olympic Games being sustainable," said John Sauven, the executive director of the UK branch of environmental organisation Greenpeace.

"When you put on any event that lasts a weekend or a fortnight, it's very hard to make it sustainable in nature, because it's a very short-term event.

"You're going to have an awful lot of people flying in for a very short period of time, consuming a huge amount of resources, and flying out again."

However, the London Games do not measure up badly, he explained, because of the transformation they have brought about in Stratford, east London, and the legacy they will leave once the Olympic flame is extinguished on August 12.

"This was a fairly polluted industrial site," Sauven said of what is now the Olympic Park.

"It has been restored, obviously a massive regeneration programme has been going on and it will be integrated into the fabric of the city, so it's not like the 2004 Athens Olympics where a lot of the facilities are just abandoned ruins now."

David Stubbs, the head of sustainability at the London Games organisers LOCOG, said: "From the beginning, sustainability was part of the project."

Some two million tonnes of polluted soil has been cleaned up and re-used on the site and rainwater is stored and re-used.

Construction materials with low carbon content were given priority in building the stadia on the Olympic Park, with the aim of reducing carbon dioxide missions by half compared with normal construction standards.

While they acknowledge the effort, Greenpeace and Friends of the Earth regret "missed opportunities" such as the scrapping of a wind turbine project on the site.

The decision made the percentage of renewable energy used on the park drop from the original target of 20 percent to 12 percent.

Above all, "corporate sponsorship of the Games has become controversial and tarnished the green image of the Games," said Sauven.

BP, heavily criticised over the Gulf of Mexico oil spill, and Dow Chemical -- which bought Union Carbide, at the heart of the 1984 Bhopal gas disaster in India -- are among the Olympic 'partners'.

The total "carbon footprint" for the Games is a far from negligible 3.4 million tonnes of carbon dioxide. Compare that with Britain's total carbon footprint for 2011, which was 549.3 million tonnes.

"We can still make improvements. Events of this nature should be carbon neutral," said Achim Steiner, executive director of the United Nations Environment Programme, who has visited the Olympic Park.

"Olympic venues and Olympic Games are a tremendous opportunity to bring a positive experience. If you have better public transport, if you have less waste, if you have clean air for the city, these are all good things that Olympic Games can accelerate."

That could be the main plank of the Games' legacy.

Poor areas of east London will inherit modern transport systems, affordable homes (48 percent of the 2,800 apartments in the Olympic village will be converted for this purpose), a major new shopping centre, sports and community facilities plus a giant new park.

Additionally, the River Lea has been cleaned up and opened up, with new bridges and pathways allowing people to cross between neighbourhoods.

"There was a dirty hole in the lower Lea Valley and it's become a biodiversity spot," said Kathryn Firth, design chief at the London Legacy Development Corporation.

The body's projects for managing what comes after the Games are ambitious: transforming the site into the public Queen Elizabeth Olympic Park by 2014, filling up to 11,000 new homes, 11 schools and nurseries, while creating up to 10,000 jobs at new "employment hubs".

And if the economic recovery doesn't follow?

"It will happen, but more slowly," said Firth.