If Mr Johnson were to be found guilty, the privileges committee could suggest sanctions including oral or written apologies, suspension from the House – which would take effect immediately – or even expulsion.

For example, in 1999 the committee suspended three MPs from the Commons – for three, five and 10 days – after they were found to have leaked committee reports.

However, the advice of the committee is not binding and so any suggested punishment would have to be put to a vote in the Commons.

This means the Government could whip its MPs to vote against introducing sanctions, although such a move would carry its own political risks.

Were Mr Johnson to be suspended for 10 days or more, he could be subject to a recall petition and ultimately lose his seat.

Sir Bernard Jenkin, Conservative MP for Harwich and North Essex

Sir Bernard Jenkin, the most senior MP on the committee, could be key to deciding the Prime Minister’s fate. Following the Labour MP Chris Bryant recusing himself as chairman for the investigation, Sir Bernard has been confirmed as his replacement.

A long-term ally of Boris Johnson, he is a Brexiteer who helped to set up Vote Leave. Although he has occasionally been critical of the Government, he is not seen as a rebel. He was first elected in 1992 and served as deputy chairman of the Conservative Party from 2005-6. He has previously held four shadow Cabinet positions as well as acting as opposition spokesman for various issues including environment, transport and constitutional affairs in the late nineties.

He is also the chairman of the liaison committee, the only one with the power to call the Prime Minister in as a witness. He took up the role after being proposed by the Government which initially led to criticism that his support for Mr Johnson would mean that he would fail to properly hold him to account. But Sir Bernard has since drawn praise for ensuring the Prime Minister did not have an easy ride when hauled in for questioning.

During a stormy Commons session in late January – which saw Andrew Mitchell, the former Cabinet minister, call for Mr Johnson to resign over partygate – Sir Bernard warned that “backbenches of the Conservative Party need no reminders about how to dispose of a failing leader”. He urged the Prime Minister to “concentrate on the fact that the country wants results” when carrying out his restructure of Downing Street.

He has not publicly criticised the Prime Minister for attending parties, telling his local newspaper in December that “where there was a party is a hypothetical question”. But he added that the “impression this has given to the public is extremely unfortunate”.

He told the Harwich and Manningtree Standard that Boris Johnson “has done the only thing he could do to bring clarity to this, he has asked the Cabinet Secretary to conduct an investigation and has apologised.” 

Sir Bernard went on to say: “But any punishment would depend on whether any law has been broken or if any rules or codes have been broken and how responsible they were judged to have been. We will just have to wait until the investigation has concluded.”

Alberto Costa, Conservative MP for South Leicestershire

Alberto Costa is a parliamentary private secretary to Suella Braverman, the Attorney General. While not officially on the government payroll, his role as a ministerial aide means he is considered part of the Downing Street political operation and would be expected to vote with the Government or resign.