It found that over 20,000 lives could have been saved in England if a full lockdown had been implemented just one week earlier in March 2020.
The report highlighted a "toxic and chaotic culture" within Number 10 under Mr Johnson and his chief adviser, Dominic Cummings.
It found that this culture led to poor decision-making, with the views of women often ignored, and Dominic Cummings was identified as a "destabilising influence".
Inquiry chair Baroness Heather Hallett stated that all four devolved governments also failed to appreciate the scale of the threat, also summarising their response as "too little, too late".