LAGOS - Nigeria's ailing President Umaru Yar'Adua will write a letter handing power over to his vice-president, his adviser has told the BBC, Saturday. The letter, formally informing the Senate that Mr Yar'Adua is on "medical vacation", automatically means his deputy becomes acting president. The president's allies have previously resisted calls for him to step aside. He has been in a hospital in Saudi Arabia since November, prompting fears of a power vacuum and street protests. The president is suffering from an inflammation of the lining around the heart and has long suffered from kidney problems. Nigeria's powerful state governors have said they believe Vice-President Goodluck Jonathan should be acting president. The BBC's Caroline Duffield in Lagos said Nigeria's political struggle over the president's fitness to rule may be nearing an endgame. The president's special adviser on legislative affairs, Abba Aji, did not say when Mr Yar'Adua would write the letter but he said there would be no "undue delay". For weeks, Nigeria has been beset with rumours and hoaxes over whether the president is alive or dead. His failure to transfer powers to his deputy before he left created a power vacuum ��" in which fear and false reports flourished, she says. Newspapers said the president was brain dead - or struck dumb. There were apparently official statements claiming the president was discharged from hospital. They later turned out not to be true. Political tensions have been high ��" government business has stalled and legislation been frozen and cracks have appeared in the uneasy peace in the oil-producing Niger Delta.