The independent sleaze watchdog advises the Prime Minister on ethics in public life, with Mr Fergusson selected to take one of the four seats on the Whitehall committee.
According to multiple reports, Mr Fergusson and Prime Minister Boris Johnson know each other from their days at Oxford University.
The appointment by the Prime Minister, made on Thursday, will see his former university colleague take up one of the four independent posts, along with Oxford politics academic Professor Gillian Peele, on the committee currently chaired by Lord Evans, the former head of MI5.
Mr Fergusson and Prof Peele's five-year terms start next month and they will be able to claim £240 for each day they work on committee business and for expenses incurred.
The UK Government insists that the Prime Minister’s decision to give Ewen Fergusson a seat on the Committee on Standards in Public Life was an "open and fair competition".
Who is Ewen Fergusson?
A graduate of Oriel College, Oxford University and the Rugby School, Mr Fergusson is the son of former British ambassador to France and Scotland international rugby player, Sir Ewen Fergusson, who passed away in 2017.
Sir Ewen Fergusson, also a graduate of the Rugby School and Oxford University, was capped five times for the Scottish international rugby team, playing against New Zealand, France, England, Ireland and Wales in 1954.
Having spent most of his career at international law firm Herbert Smith Freehills, Mr Fergusson was formerly a partner in the firm’s finance division from 2000 to 2018, according to his official Government biography.
This also states that he was ‘ranked consistently as one of the City of London’s leading individual lawyers in his sector’ and that he is a non-magistrate member of the Lord Chancellor’s advisory committee for south-east England.
Mr Fergusson attended Oxford University to read History from 1984 to 1987 and features in a famous 1987 picture of Bullingdon Club members wearing contrasting black and white three-piece suits and bow ties while at university, with Mr Johnson in the front row and David Cameron stood towards the back.
What have opposition parties said about Mr Fergusson’s appointment?
Deputy Labour leader Angela Rayner has called for all correspondence with No 10 relating to Mr Fergusson's appointment to the advisory committee to be published.
"This is more of the same Conservative cronyism. This Prime Minister does not even care to hide it," said the shadow chancellor of the Duchy of Lancaster.
"The Government must publish all the correspondence between the Cabinet Office, the panel and Downing Street relating to this appointment.
"If it does not, it will confirm the suspicion that they think there is one rule for them and another for everyone else.
"Labour would clean up politics, starting with a single ethics and integrity commission with the power to oversee and enforce anti-corruption and ethics laws and regulations."
A Cabinet Office spokesman said: "Mr Fergusson applied through open and fair competition, following the Governance Code for Public Appointments.
"His application was carefully considered on its merits by the Advisory Assessment Panel, which interviewed him and found that he was appointable."
Additional reporting by PA Political Correspondent Patrick Daly.