Nigeria midfielder John Mikel Obi has been ruled out of the 2010 World Cup in South Africa with a knee injury.

Mikel joins fellow Chelsea stars Michael Essien and Michael Ballack in missing out through injury, while Didier Drogba is a major doubt.

The 23-year-old underwent minor knee surgery in May and feels he has not recovered sufficiently.

"We have dropped Mikel from the squad after he said he did not want to risk his career," said a Nigeria spokesman.

Nigeria will replace Obi Mikel with the uncapped Brown Ideye, a striker from French club Sochaux.

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Mikel has been capped 30 times for his country and was the last player to join up with the rest of the squad at their training camp in Essex.

In South Africa, Mikel cut short his workout with the team on two consecutive days. He also missed the team's two build-up games against Saudi Arabia and Colombia.

On Friday he trained with the rest of the squad for the first time since they arrived at their World Cup base in Durban, but later announced he was not fit enough to play in South Africa.

Nigeria have been drawn in Group B alongside two-time world champions Argentina, former European Championship winners Greece and 2002 World Cup semi-finalists South Korea.

They play a final friendly against North Korea in Johannesburg on Sunday before starting their World Cup campaign against Argentina on 12 June.