The reshuffle aims to make May’s government more diverse and youthful in addition to bringing in more women.
She has been forced to make changes following two resignations and one sacking.
Sir Michael Fallon, the defence secretary, resigned in November 2017 amid allegations around his conduct.
One week later, international development secretary Priti Patel quit after a row over unauthorised meetings with Israeli officials.
And in December, first secretary of state Damian Green was sacked over his “misleading statements” about the discovery of pornographic images on his Commons computer during a police raid.
According to sources, except for the Cabinet big beasts, Education Secretary Justine Greening, Business Secretary Greg Clark and Commons Leader Andrea Leadsom are likely to be sacked or demoted.
Chancellor Philip Hammond, Foreign Secretary Boris Johnson, Brexit Secretary David Davis and Home Secretary Amber Rudd will stay in place, according to reports.
A minister added, “Some ministers have been around for years and not going anywhere.
“This reshuffle will show how emboldened she is now. She is unlikely to touch the big beasts but the question is how far will she go below that.”
The changes will also enable May to promote Tory MPs who were elected in 2015.