He said renewed lockdown would “result in more life-years lost than it hopes to save” and argued that “building immunity” would be more effective.
The party leader also said there should be “focused protection” from coronavirus for the vulnerable.
But Boris Johnson will tell MPs later there is “no alternative” to lockdown.
From Thursday, pubs, restaurants, gyms, non-essential shops and places of worship in England will close for four weeks, in an effort to stem the increase in infections.
The House of Commons vote on the measures on Wednesday, with the prime minister outlining his plans to MPs on Monday.