Duncan Smith will be in charge of meeting the Conservative pledge to find £12bn worth of savings in the welfare budget by 2020. Only £2bn of that was accounted for in their manifesto.
The Prime Minister will also say that Tina Stowell will stay on as leader of the House of Lords. Stowell will be promoted to become a full member of the cabinet, and receive a full cabinet salary. This did not happen in the previous parliament.
These announcements follow the reappointment of George Osborne, Theresa May, Michael Fallon and Phillip Hammond to roles they held in the previous government.
After last week’s shock election victory, Cameron has also made Michael Gove the justice secretary, instead of Chris Grayling, who is now leader of the House of Commons. Mark Harper, who previously had to resign as immigration minister, is now chief whip. The role will be critical, given Cameron’s razor-thin parliamentary majority.
The full Cabinet will be announced formally today. Tory MPs Sajid Javid and Matthew Hancock are tipped as front-runners for his role of business secretary and the cabinet is expected to include a number of highly-regarded women such as Amber Rudd and Andrea Leadsom.