The government has published a letter from Boris Johnson's former ethics adviser Lord Geidt who resigned suddenly without explanation.
Lord Geidt was known to be unhappy with the prime minister's handling of Partygate and had suggested he may have broken the ministerial code.
In his letter, he says he had been asked to consider another matter that could have breached the code.
Downing Street has also published the PM's reply to Lord Geidt.
A friend of Lord Geidt told the BBC they feel "deeply saddened that such an honourable and dedicated public servant has been put in a position where he feels forced to resign".
The government announced it would publish the letters minutes before minister Michael Ellis was called to the Commons to face an urgent question on the subject.
Labour shadow minister Fleur Anderson said the prime minister had "driven both of his own hand-picked ethics advisers to resign in despair" and added that, without reform, the position was an "unworkable".
William Wragg - Conservative MP and critic of the prime minister - asked how long it would take to find a new ethics adviser, noting that last time the recruitment process took five months.
Mr Ellis said the Government was "disappointed" by the resignation but added that the letters from both men would "speak for themselves".
In recent days Lord Geidt had expressed frustration over the handling of the Partygate row.
He said it was reasonable to suggest the prime minister may have breached the ministerial code by breaking Covid lockdown laws.
Lord Geidt has previously been critical of the prime minister over an investigation into renovations in the Downing Street flat.
He is the second ethics adviser to quit under Mr Johnson's premiership. Sir Alex Allan, who resigned from the role in 2020 after Mr Johnson chose not to accept his finding that Home Secretary Priti Patel had bullied civil servants.
Who is Lord Geidt?
- Born in 1961, Christopher Geidt is a former army intelligence officer who later worked as a diplomat in Sarajevo, Brussels and Geneva
- In 2002, he began working for the Royal Household and served as the Queen's private secretary for 10 years from 2007
- He stepped down after a "power struggle" between Buckingham Palace and the Prince of Wales, the Times reported in 2017
- Lord Geidt is chairman of King's College London and also chairs a board of the investment firm Schroders
- He lives on the Isle of Lewis in the Outer Hebrides, where he grew up and where he now has a sheep farm. He is married with two daughters
Lord Geidt's job involved providing advice to Mr Johnson about the ministerial code. He could also investigate ministers for breaching the code - but only if asked to do so by the prime minister.
The code, which outlines the rules ministers must follow, says there is an "overarching duty" on them to comply with the law. If the code is broken, ministers are expected to resign.
Questions have been raised over whether Mr Johnson breached the code after he was fined in April for attending a birthday party in Downing Street in June 2020 - when Covid restrictions were in place.
Despite receiving the fine, Mr Johnson said he would not resign and in a letter to Lord Geidt he said there had been "no intent" to break Covid regulations.
It had been reported that Lord Geidt threatened to quit last month after the publication of a report into lockdown breaches in Downing Street unless Mr Johnson issued a public explanation for his conduct.