Boris Johnson is expected to name Prince William's former aide as the new head of the civil service.
Simon Case's appointment as Cabinet Secretary, which was due to be announced on Tuesday, comes amid efforts by No10 to shake-up Whitehall, according to the FT.
Mr Case, 41, was the Duke of Cambridge's right-hand man for nearly two years before moving to Downing Street to help with the coronavirus crisis.
His predecessor Sir Mark Sedwill announced he would step down earlier this year after reports of clashes with Boris Johnson's top aide Dominic Cummings.
Mr Cummings has been pushing for reform the civil service and reportedly told political advisers in June that a "hard rain was coming" to Whitehall.
Existing tensions between the civil service and No10 have deepened during the pandemic, with five permanent secretaries leaving their posts over the past six months.
Jonathan Slater, the top civil servant at the Department for Education, was sacked last week following the exams fiasco - only days after Sally Collier quit her post as head of the under-fire exams regulator Ofqual.
However no ministers have resigned over the Government's handling of the coronavirus crisis.
Dave Penman, general secretary of the FDA union, which represents civil servants, said: "Congratulations to Simon Case, a big role at a challenging time, but then it always is.
"He clearly has the confidence of the PM - vital if he is to improve No.10’s relationship with the civil service."
Mr Case served under David Cameron and Theresa May before going to work for the Duke of Cambridge in 2018.
His surprise appointment comes after speculation the job would go to a more experienced Whitehall official, such as Chris Wormald, the permanent secretary at the Department of Health and Social Care.
Charles Roxburgh, a senior Treasury official, and Antonia Romeo, permanent secretary at the Department for International Trade, were also thought to be in the frame for the powerful post.
The Cabinet Office refused to comments on reports of Mr Case's appointment.
A spokesperson said: "An official announcement on the new Cabinet Secretary will be made on Tuesday, 1 September."
