Keir Starmer warned Britain will face a "long and bleak winter" unless Boris Johnson uses this month to halt a devastating second spike of coronavirus.
The Labour leader said there is "precious little evidence" of serious preparation for a resurgence in Covid-19 cases or an acceptance from ministers that the track and trace system is still not functioning properly.
Writing for the Guardian, Starmer called for mass testing of asymptomatic people and a clear plan setting out what "hard decisions" Johnson is prepared to take in order to keep schools open if cases rise over coming weeks.
With parts of the north of England under local lockdown, the UK Government's chief medical officer, Chris Whitty, warned the limits of easing lockdown might already have been reached.
The Children’s Commissioner for England, Anne Longfield, warned on Tuesday that if the choice has to be made in a local area about whether to keep pubs or schools open "then schools must always take priority".
English schools minister Nick Gibb claimed central government cannot decree that schools must be prioritised over pubs, saying the decision will be left to local health experts.
But Starmer said the nation is at a "crucial point" in the pandemic.
He blasted the UK government's lack of clear communication throughout the crisis, and said its Test and Trace programme needs rapid improvement.
He added the Prime Minister must admit his "world-beating" £10 billion system is flawed.
Starmer said: "His repeated refusal to accept that test and trace isn’t functioning properly is a roadblock to fixing the issues and restoring public confidence."
He wrote: "On the occasions that the government has acted at pace, it has too often done so without a clear plan.
"But the reality is that if the government doesn't use this summer wisely, focusing on driving down the rate of infection, Britain faces a long and bleak winter."
