The Prime Minister gathers his new team in Downing Street shortly after 10am as he attempts to move on from the dramatic resignation of his former chancellor Sajid Javid.
However Mr Javid's resignation has thrown this into disarray.
Asked whether the planned Budget statement would still take place on March 11, he said only: "Extensive preparations have already been carried out for the Budget and they will continue at pace".
Sunak previously served as the Chief Secretary to the Treasury, the second in command to the finance minister.
Irish Prime Minister Leo Varadkar led tributes to Smith's efforts, saying he was "one of Britain's finest politicians of our time".
The government said Thursday that Alok Sharma, previously in charge of global development, would become business secretary and also take charge of COP26.
"The Prime Minister said he had to fire all his special advisers and replace them with Number 10 special advisers to make it one team".
The BBC's Business Editor Simon Jack said the prime minister suspected Javid might resign if pushed to get rid of his advisers and he described Sunak as an "oven-ready" Chancellor.
Mr Jenrick denied that newly-appointed Chancellor Rishi Sunak would be the PM's "puppet" after accepting terms of office that his predecessor rejected.
Along with the ministerial exits, the reshuffle - which Downing Street insiders had predicted would be "conventional" before the row with Mr Javid - included promotions for MPs who are highly rated by Number 10. Sunak, the son-in-law of Infosys co-founder Narayana Murthy, will join Home Secretary Priti Patel on the top government bench as the Chancellor of the Exchequer.
Johnson had earlier sacked Northern Ireland secretary Julian Smith, a surprising move given Smith's role in restoring the power-sharing government to Belfast last month after a three-year suspension.
Ultra-loyal Alok Sharma, a former minister for global development, was appointed as the new minister for business and also the head of the COP26 climate change summit, when world leaders will travel to Scotland in November.
Oliver Dowden has become a full cabinet member as culture secretary having previously attended the meetings as paymaster general.
Foreign Secretary Dominic Raab, Johnson's de facto deputy Michael Gove and several other key ministers stayed in the post - and Javid was widely assumed to be safe.