The cost of corruption in Afghanistan rose sharply last year to £2.5 billion, and half of all Afghans bribed public officials for services, according to the UN.

The findings came despite repeated promises by President Hamid Karzai to clean up his government.

The international community has long expressed concern about corruption in Afghanistan because it reduces confidence in the Western-backed government.

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Mr Karzai outlined steps for his ministries, prosecutors and judiciary to fight bribery, nepotism and cronyism last July.

But a survey by the UN Office on Drugs and Crime and Afghanistan's anti-corruption unit showed a slight improvement in curbing the common practice of paying bribes for public services.

Fifty per cent of the adult population paid at least one bribe to a public official in 2012, a 9% drop from 2009, according to the study, based on interviews with 6700 adults.