Head of the Civil Service of the Federation, Mrs Winifred Oyo-Ita, has sent her resignation letter to President Buhari.
EFCC is investigating her over alleged N3billion contract scam, abuse of duty tour allowance, money laundering, among others.
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 has been deployed to monitor her movement in the Abuja hospital.
EFCC recently questioned Oyo-Ita over allegations that she used front companies to get her contracts when she was a permanent secretary before her appointment as Head of Civil Service by President Buhari back in 2016.
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.