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LAGOS (Reuters) - Nigeria is on the brink of constitutional crisis with its ailing president not transferring powers to his deputy and political king makers feuding over his succession, a senior lawyer and a former U.S. envoy have warned.

President Umaru Yar'Adua has been absent from Africa's most populous nation for more than a month receiving treatment for a heart condition in Saudi Arabia, but there have been no official updates on his health for weeks.

Vice President Goodluck Jonathan has been presiding over cabinet meetings but executive powers have not officially been transferred to him, leading to questions over the legality of decisions made by the government in Yar'Adua's absence.

The Nigerian Bar Association (NBA), the umbrella body of all lawyers in the country, has launched legal action against the Attorney-General, asking a top court to declare that Yar'Adua has violated the constitution by omitting to transfer powers.

"We are saying there is a duty on the president to do it, it is not discretionary ... We cannot continue this way, we are not running a banana republic," NBA president Rotimi Akeredolu told Reuters in a telephone interview on Thursday.

"We are treading a very dangerous path and we have to be very careful. We are trying to paper over a few cracks ... but if we are not careful the whole building will collapse," he said.

The NBA's legal action follows a similar suit already brought by prominent human rights lawyer Femi Falana.

It also adds to a crisis in the judiciary triggered by the swearing in on Wednesday of a new chief justice, the first time in the country's history the head of state has been absent for the ceremony and an act some senior lawyers say is illegal.