The Covid public inquiry has asked to see Boris Johnson’s WhatsApp messages when he was prime minister, alongside communications with other senior officials.
Counsel for the inquiry, Hugo Keith KC, said thousands of documents had been requested to inform the inquiry.
He said: "We’ve asked for ministerial submissions, Number 10 daily briefing documents, records of written and oral advice to ministers and details of internal communications including a WhatsApp group, which included the prime minister, Number 10 and other senior officials.”
Initial responses from government departments indicated tens of millions of documents could potentially be relevant with reviews of documents in the Cabinet Office alone estimated as likely to take over three years.
Mr Keith said the inquiry is instead taking a "targeted approach", by seeking documents relevant to the key narrative events.
The looming inquiry recently played a key role in the recent leadership election when some Tory MPs feared it would not cast Mr Johnson in a flattering light.
This led to some saying Mr Johnson should not return to No10.
Earlier, Mr Keith said he would examine whether lives could have been saved by earlier lockdowns.
Inquiry chairwoman Baroness Heather Hallett will examine the effectiveness of mandatory lockdowns in controlling the spread of coronavirus, the inquiry was told.
Want a quick and expert briefing on the biggest news stories? Listen to our latest podcasts to find out What You Need To know
This will include “the relationship between the timeliness and the length of the lockdown, and the trajectory of the disease”, Mr Keith said.
He continued: “How were economic and societal impacts, including the impact on physical health, healthcare provision, mental health, education and societal wellbeing, assessed and weighed in the balance?
“And perhaps.... the single most important question: is it possible to say what the likely effects of earlier or different decisions to intervene would have been? The counterfactual proposition.
“Bluntly, would lives have been saved if the lockdowns had been imposed earlier or differently?”
Around 200 scientists, including all those involved in the Sage group and others in the Independent Sage group, have been asked to give evidence about the effectiveness of the pandemic response.