Former adviser Dominic Cummings is to be questioned by MPs about the government’s response to the pandemic.
Mr Cummings, who left Downing Street last autumn, has been highly critical of the government in recent weeks.
He is also expected to claim later that Boris Johnson was keen to visit the Queen in person five days before the first Covid lockdown began.
Downing Street has denied this happened and said their conversation on 18 March last year took place on the telephone.
The Queen went from Buckingham Palace to Windsor Castle shortly afterwards.
This happened in order to protect the then 93-year-old monarch from the danger of catching coronavirus.
Mr Cummings will appear before a joint parliamentary committee looking into the handling of the pandemic.
He has already claimed that “secrecy” within Whitehall “contributed greatly to the catastrophe”.
Ahead of his meeting, Mr Cummings tweeted a picture which he called the “first sketch of Plan B, PM study, Fri 13/3 eve – shown PM Sat 14/4”.
It appears to show a whiteboard with sketches from attendees at the meeting. Mr Cummings promised to reveal further details later.
The questioning of Mr Cummings will take place at a joint session of Parliament’s health and social care and the science and technology select committees.
Mr Cummings – who gave evidence to one committee in March – will be grilled by MPs at 09:30 BST for around four hours, with live coverage taking place on the BBC News website.
Questions are expected to cover a range of topics including decision-making in the early months, the timings of lockdowns and other restrictions and the procurement processes.
It is likely that Mr Cummings will also face questions about his controversial trip to Barnard Castle during the first lockdown in 2020, when he said he wanted to check his eyesight before driving back to London.
Last week, he tweeted the government’s pandemic plan had been “part disaster, part non-existent” and he called for more scrutiny of policy to ensure the Indian variant is dealt with.
And in March, he described the department of health as a “smoking ruin in terms of procurement and PPE (personal protective equipment)” at the start of the pandemic.
‘Captain of hindsight’
Ahead of Mr Cummings’s committee appearance, Transport Secretary Grant Shapps said he would “leave others to judge how reliable a witness that former adviser happens to be”.
“I think most people will have come to their own conclusions a long time ago about [this] particular adviser and the reliability of statements and the rest of it,” he told BBC Breakfast.
“There was no manual to deal with a pandemic – and it’s easy to be captain of hindsight on these things but we had to work on things day by day,” he added.
Dominic Cummings was in the room when decisions about lives and deaths were made during the Covid emergency.
He’s made no secret of his frustration now, at the speed of decision-making when the virus had arrived in the UK, blasting the level of preparations and the government’s original plan.
The former adviser’s real fire today is expected to be turned on the prime minister’s attitude to bringing back restrictions in September when Covid was again taking hold, but No 10 did not act until later on.
It is understood he will share his belief that the failure to toughen the rules led to a much bigger outbreak of the disease.
And he will suggest that was Boris Johnson’s terrible mistake because the government by then had a much better understanding of the virus, could have predicted what would happen and could have prevented much of it.
Yet Mr Cummings cannot extricate himself from what went wrong.
Few others had more influence over the decisions that were made.
More than 127,000 people diagnosed with coronavirus have died in the UK since the start of the pandemic, but 72% of the adult population has had one vaccine jab and 44% have had two doses.
According to the government’s “roadmap”, the remaining restrictions on social contact in England are set to end on 21 June.
The government says it was always guided by the “best scientific data”.
A Downing Street spokesman said: “There is a huge task for this government to get on with.
“We are entirely focused on recovering from the pandemic, moving through the roadmap and distributing vaccines while delivering on the public’s priorities.
“Throughout this pandemic, the government’s priority has been to save lives, protect the NHS and support people’s jobs and livelihoods across the United Kingdom.”
This session is not part of a proposed independent public inquiry into the handling of the pandemic, which the prime minister says will be held in spring 2022.