They said it collected more than 52 tonnes of scrap metal from guns, knives and old keys in the last financial year.

The force said the recovered steel, iron and other valuable metals were melted down and used in construction, with some of the materials going to the Olympic site in Stratford, East London.

Sarah Foster, a spokeswoman for the Metropolitan Police, said: "We're constantly looking for new ways to recycle and new markets for our recyclable materials, to conserve resources, save energy and money."

The Met's annual environment report said the force recycled a total of 3,182 tonnes of waste in 2008-9, including uniforms, firing range bullets and horse manure.

Foster said: "The Met is a huge operation across the capital, employing over 50,000 people working from almost 800 buildings.

"Our environmental policy has been really successful in minimising the potential environmental impact of what we do for London."