The former premier criticised Johnson’s use of modelling by scientists which predicted as many as 4,000 deaths a day to back up his decision for a lockdown until December 2.
She also called for more data on mental health, domestic abuse and the economy, and accused the PM of selecting data to show the public that supported his policy decision.
But before the Maidenhead MP got a chance to confront her successor during a debate in parliament, he got up and left the chamber.
May, who looked bemused by Johnson’s exit, told the Commons the graph used by chief scientific officer Patrick Vallance and chief medical officer Chris Whitty was flawed.
She said the trajectory outlined in the graph would have meant 1,000 deaths were being recorded every day in late October, which was not the case.
“So the prediction was wrong before it was even used,” she said, addressing her concerns to health secretary Matt Hancock, who was on the frontbench.
“And this leads to a problem for the government – for many people it looks as if the figures are chosen to support the policy rather than the policy being based on the figures.
“We need these proper analyses. We need to know the details behind these models. We need to be able to assess the validity of those models.”
May also raised concerns about a lack of data on the cost of the government’s Covid-19 decisions, including on mental health, domestic abuse, non-Covid-19 treatments, “possibly more suicides” and to the economy.
She told MPs: “Jobs lost, livelihoods shattered, businesses failing, whole sectors damaged. What sort of airline industry are we going to have coming out of this? What sort of hospitality sector? What sort of small independent shops will be left?
“The government must have made this analysis, made this assessment: let us see it and make our own judgements.”
Asked if the government would provide analysis of the impact of lockdowns, as requested by May, No.10 said the implications were “considered at every stage” of the decision-making process.
The prime minister’s official spokesperson said: “The prime minister, the chancellor and the government have been clear throughout about the cost, not just to the economy from the measures we’ve been forced to put in place, but also the impact they have on mental health, and both of those things have been considered at every stage when we’ve been deciding what action we need to take.
“In the future, as we consider future measures required, that will continue to be the case.
“We fully understand the toll which this virus is taking on livelihoods but also people’s mental health, and that’s why we’ve provided packages of support for mental health groups to ensure people can access the help they need.”
MPs are due to vote on the new lockdown, which comes into force on Thursday, with a small number of “lockdown-sceptic” Tory MPs expected to rebel.