Corruption is a “serious problem” in 40 of sub-Saharan Africa’s 46 states, says an anti-corruption watchdog.

Transparency International (TI) says it has seen no improvement in powerhouses Nigeria and South Africa.

Its corruption index puts Somalia at the top of the list of the world’s most corrupt countries.

The annual index looks at factors such as the prevalence of bribery and the perception that government officials go unpunished for corruption.

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The watchdog said the countries perceived to be the most corrupt tend to be in conflict; have weak institutions such as the police and the courts and lack independent media.

Somalia has not had an effective central government since its long-serving ruler, Siad Barre, was overthrown in 1991.

North Korea, which has been one of the world’s most secretive societies, shared the spot of most corrupt with Somalia.

The “cleanest” countries, such as Denmark, Finland and Sweden, tend to show the public how money is spent and have judges that don’t differentiate between rich and poor, the report says.

TI picked out Ghana – which has been rocked by an undercover film showing judges allegedly taking bribes – as a pocket of hope where activists “worked hard to drive out the corrupt”.

Senegal, where the government has introduced a series of anti-corruptions laws, was one of the biggest improvers this year, TI said.