Head of the Civil Service of the Federation, Mrs Winifred Oyo-Ita, has sent her resignation letter to Pres. Buhari.
Last week, her international and diplomatic passports were seized by the agency and she has been restricted to the country pending the conclusion of investigation.
Although she is hospitalized having complained of being ill, sources said security operatives had been deployed to monitor her movement in the Abuja hospital.
EFCC recently questioned Oyo-Ita over allegations that she used front companies to get contracts when she was a permanent secretary before her appointment as head of service by Buhari in 2015.
It was also reported that N600 million was traced to the account of one of her aides who has been quizzed by the anti-graft agency and released. However, Oyo-Ita has denied the allegations.