Video report by ITV News Political Correspondent Paul Brand
The UK's top civil servant is set to face questions amid allegations against Home Secretary Priti Patel and the prime minister's chief adviser Dominic Cummings, ITV News understands.
Mark Sedwill, who is Cabinet Secretary and Head of the Civil Service, will appear in front of the Public Administration and Constitutional Affairs Committee on Tuesday.
ITV News Political Correspondent Paul Brand says Mr Sedwill is expected to "face a barrage of questions" relating to allegations against Ms Patel and Mr Cummings.
It comes after Boris Johnson gave his firm support to the home secretary saying he is "sticking by her" despite the emergence of fresh bullying allegations against her - a topic which dominated Prime Minister's Questions (PMQs).
Labour leader Jeremy Corbyn criticised the PM's support for Ms Patel and demanded an "independent" investigation be held into her conduct.
He said Mr Johnson has "no shame in defending bullying" in his Government and pointed to "further allegations" against the home secretary, which emerged overnight from when she ran the Department for International Development.
Mr Johnson failed to confirm if he knew about bullying allegations which had been levelled against her, before he appointed her as Home Secretary.
Mr Corbyn said the latest allegations that Ms Patel bullied staff in a third government department would suggest a "shocking and unacceptable pattern of behaviour" if they were found to be true.
But Mr Johnson offered his support to Ms Patel, who was sat on the Government front bench for Prime Minister's Questions, before attacking Labour for its handling of bullying claims.
The latest bullying claims come after the resignation of the top civil servant at the Home Office, Sir Philip Rutnam, who claimed he had been the victim of a coordinated campaign of abuse in which he said Ms Patel was complicit.
Mr Johnson said he has "every confidence" in her as home secretary but said any allegation should be "properly investigated by the Cabinet Office and that is what is happening."
But Mr Corbyn said the "government cannot be judge and jury over its own conduct" and demanded "an independent element to that investigation".
When asked if Ms Patel should resign if the investigation finds she did in fact bully people, Mr Johnson said "the home secretary is doing an outstanding job", adding "I'm sticking by her".
A senior official at the Department for International Development (DFID) reported a "tsunami" of allegations of abuse by officials in her private office when Ms Patel worked as secretary of state, BBC 2's Newsnight reported.
A former aide at the Department for Work and Pensions is also reported to have received a £25,000 payout from the Government after claiming she was bullied by Ms Patel when she was employment minister.
A spokesman for Ms Patel said: “The Home Secretary categorically denies all of these allegations.”
In PMQs Jeremy Corbyn asked: "Was the Prime Minister aware of these allegations? And if he was, why did he appoint her?"
Mr Johnson replied: "The Home Secretary is doing an outstanding job, I have every confidence in her. If there are allegations, of course it's right they should be properly investigated by the Cabinet Office and that is what is happening."
He added he would "take no lessons about bullying" from the leader of a party where "female MPs were bullied so badly in the manner of anti-Semitism they actually left the party".
Meanwhile, Kane Blackwell, a former paid intern for Ms Patel who worked in her Parliamentary office from 2017 til 2018, told ITV News she was a "compassionate" and "not a bully".
"Priti was probably one of the best bosses I've ever had to work for," he said.
"She was compassionate, kind, considerate to me and others, she was never rude, aggressive, or anything like that."
He believes allegations against the home secretary come from a "concerted effort in the establishment" who do not like her.
European Research Group (ERG) chair Mark Francois on Wednesday morning said Ms Patel is "an excellent home secretary who's doing what we were elected to do".
On the allegations of bullying, Mr Francois asked for "balance" and pointed to Jeremy Corbyn's peerage recommendation for John Bercow, who is also facing allegations of bullying.
A Tory source was quoted on Newsnight as saying there was a "concerted effort" by section of the civil service to undermine Ms Patel.
They added that "dark forces" were trying to influence a Cabinet Office inquiry into her conduct.
The latest allegations relate to 2017 when Ms Patel was sacked by Theresa May as international development secretary over unauthorised contacts with the Israeli government.
After her dismissal, a senior figure in DFID was said to have approached staff in her private office about allegations of bullying.
The senior person was said to have been told about multiple claims by staff of being humiliated and coming under heavy pressure in emails - similar allegations made about Ms Patel's conduct at the Home Office.
The senior figure then went to see another senior figure in DFID and urged them to contact the then Cabinet secretary - the late Sir Jeremy Heywood – so that her conduct recorded was "in the system" if she ever returned to government.
Although the person making the claims has not been named, they were said to be prepared to give evidence to the Cabinet Office inquiry currently under way into allegations she broke the ministerial code.
The individual concerned was also said to be ready to give evidence in support of Sir Philip – who is claiming constructive dismissal – if his case goes to an industrial tribunal.
A Tory source, quoted by Newsnight, hit back, saying: “What we are seeing is a concerted effort by certain sections of the Civil Service to undermine a Home Secretary trying to deliver what people want on crime and immigration.
“It is deeply disturbing that dark forces are trying to influence the findings of a Cabinet Office inquiry.
On Tuesday, Ms Patel expressed her “regret” at the decision of Sir Philip to quit and urged staff in the Home Office to come together as “one team”.
In a joint email to staff written with Sir Philip’s interim successor Shona Dunn, she insisted they cared about the wellbeing of all employees.
They expressed their gratitude to the staff for their continuing hard work and “commitment to deliver the Government’s priorities”.
Number 10 said the Cabinet Office investigation would be a "robust process" to establish the facts.
A spokesman indicated the investigation would look at issues beyond Sir Philip's claims: "There are established process for civil servants to raise issues with their line manager and through the permanent secretary of their department.
"Any issues raised can be passed to the Cabinet Office."