Colombia's Constitutional Court has blocked conservative President Álvaro Uribe from seeking a third term in office in May, ruling unconstitutional a law calling for a referendum on the issue.

President Uribe, 57, was first elected in 2002 and re-elected in 2006 after his supporters in parliament amended the constitution to allow him to run for a second term.

Chief Justice Mauricio González said that the law calling for a referendum on Mr Uribe's candidacy "constitutes a grave violation of democratic principles".

Mr Uribe, a conservative with near 70-percent approval ratings and Washington's closest ally in South America, says he will honour the ruling, which cannot be appealed.

During his eight years in power, President Uribe has become very popular for his US-backed war on leftist rebels and cocaine traffickers.

More than 80 percent of Colombians said in a December poll they would vote for Mr Uribe if he ran for a third term.

Presidential elections will be held on 30 May.