Has Nigeria's First Lady seized control of the nation?

Mounting anger in Nigeria at the mystery surrounding the health of the country's president has led many to question who is really in charge of the African giant: its elected leader, the acting president or the first lady?

Turai Yar'Adua, a former schoolteacher and long-time wife of the president, is accused of making a power grab and taking charge of the oil-rich nation along with a cabal of advisers.

The 53-year-old first lady has denied all access to her husband since his night-time return to Nigeria last week, following a three-month absence at a hospital in Saudi Arabia. Three attempts to visit Umaru Yar'Adua – whose medical condition has not been publicly explained – by the acting president Goodluck Jonathan have been rebuffed.

The 67-year-old president has not been seen in public since November and is believed to be so critically ill that senior sources believe it is "highly unlikely" he can recover. After months of emergency treatment for what was supposed to be pericarditis, Mr Yar'Adua was brought home and kept in the grounds of the presidential villa inside an ambulance for nearly a week while an intensive-care unit was hastily built inside.

The only people believed to have seen him are his wife, the family's head of security and the Saudi medical team which has accompanied him back to Nigeria. The strict secrecy over the president's health and absence of any formal authority on the part of the first lady to restrict access to the head of state has prompted one law firm in Abuja to sue her and her kitchen cabinet of advisers for violating the constitution.

Africa's most populous country has been destabilised by a prolonged power vacuum which has endangered peace efforts in the Niger Delta, has frozen badly needed reforms and has forced Nigeria's convoluted power struggles into the public domain. The confusion has also threatened the gentlemen's agreement in the ruling party under which the majority Muslim North and predominantly Christian South take turns in the presidency.

The mother of seven has been accused by some in the media of launching the "Turai coup", of plotting to take the vice-presidency and by others of endangering her husband's health in a desperate bid to hold onto power. She has, however, made no public statement thus far.

Attention has turned already to the race for the vice-presidency in the event that Mr Jonathan formally takes the top job. Mrs Yar'Adua is thought to want the job for herself or for one of her two sons-in-law who are already state governors.

Whoever takes the vice-presidency ahead of next year's election would then be the frontrunner for the top job, analysts said.

The first lady has been able to see off attempts this week to limit her husband's remaining authority despite his medical condition, as she won the backing of Nigeria's powerful state governors.

Turai's apparent skill at the political game in the capital Abuja has prompted a re-examination of a first lady previously thought to be a modest successor to the brash senior wife of Olusegun Obasanjo who came to be known as "greedy Stella".

Mrs Yar'Adua was born to a modest family in the north state of Katsina and educated at a government school. She met and married the man who would be president when he was still a chemistry teacher.

But her new family, the Yar'Aduas, was already part of the political elite in Nigeria even though her husband was seen as the least likely member to enter politics. His elevation to the presidency was a surprise, especially as he had spoken of his desire to return to teaching. His elder brother, Major General Shehu Musa Yar'Adua, was a party leader and powerful figure. When Yar'Adua senior was asked to endorse his younger brother's bid to be governor of Katsina State he is said to have asked who really wanted the job, Umaru or Turai?

Because of her husband's persistent health problems, family friends and colleagues remember her playing a significant role in the running of the state.