Sir Michael Fallon resigned last night as Defence Secretary in a shock announcement.
Prime Minister Theresa May has appointed Julian Smith as the new Chief Whip and Esther McVey has been appointed Deputy Chief Whip.
When he announced his resignation, Sir Michael said his conduct had “fallen short” of the required standards of a government minister.
MPs Tobias Ellwood and Penny Mordant were also tipped to take on the top role.
Mr Williamson’s appointment has been a shock to many in Westminster as he was not considered to be a frontrunner for the job.
Mr Williamson was David Cameron’s parliamentary private secretary and was awarded a CBE for his work after the former PM resigned.
Sir Michael’s resignation came after it emerged that he had repeatedly put his hand on journalist Julia Hartley Brewer's knee at a dinner in 2002.
However, the journalist believes other allegations from the former minister are expected to emerge.
She said: “All the people I’ve spoken to in and around the Conservative party and Downing Street seem to make clear that they expect other allegations to surface.