The allocation of financial affairs responsibilities to Mr Niwatthamrong was agreed at a meeting of the remaining caretaker cabinet members at the Office of Permanent Secretary for Defence in Muang Thong Thani on Thursday, the first since Yingluck Shinawatra was dismissed on Wednesday by the Constitutional Court over the illegal transfer of National Security Council secretary-general Thawil Pliensri in 2011.
Mr Niwatthamrong will also takeover as chair of the National Rice Policy Committee and oversee defence affairs while the government is seeking advice from the Council of State on whether Ms Yingluck can retain the defence portfolio, Mr Phongthep said.
The cabinet is uncertain whether Ms Yingluck can continue in her position as the defence minister, a position she assumed after ordering Mr Thawil's removal to an inactive post.
Ms Yingluck had been concurrently premier and defence minister since July last year.
Mr Kittirat's work as finance minister, a position he held concurrently, was delegated to caretaker deputy minister Tanusak Lek-uthai, Mr Phongthep said.
Caretaker Deputy Prime Minister Yukol Limlaemthong was directed to take over the job vacated by former deputy premier Plodprasob Suraswadi, and Mr Phongthep will take over the work of former deputy prime minister Pracha Promnok and foreign minister Surapong Tovichakchaikul, including foreign affairs and supervision of the Centre for the Administration of Peace and Order.
Suranand Vejjajiva was appointed prime minister's secretary-general and Pol Maj Gen Thawat Boonfueng the deputy secretary-general.
All new assignments still need the stamp of approval from the Election Commission.
The cabinet was undecided on who would represent Thailand at the Asean summit in Myanmar this weekend.
Mr Kittiratt, Mr Plodprasob, Pol Gen Pracha Promnok and Mr Surapong were among the nine serving ministers also disqualified from the cabinet by the court because they were members of the Yingluck cabinet that removed Mr Thawil from the security agency in 2011.