Andrew Lloyd Webber has said he will open his theatres on 21 June, and is prepared to be arrested if authorities try to intervene.
The composer has also warned he may have to sell his six West End venues if the government does not relax its restrictions as set out in its roadmap.
The pandemic has had a catastrophic financial impact on the theatre industry and many have remained closed despite the ease in COVID-19 restrictions as it is not financially viable for them to open with reduced capacities.
Lord Lloyd Webber is preparing for a production of Cinderella, which is scheduled to open for previews on 25 June ahead of its world premiere in July.
"We are going to open, come hell or high water," Lord Lloyd Webber told the Daily Telegraph.
Asked what he would do if the government postponed lifting lockdown, he said: "We will say: 'come to the theatre and arrest us.'"
The 21 June "freedom day" is in doubt due to concerns over the impact of coronavirus variants.
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Lord Lloyd Webber said scientific evidence shows theatres are "completely safe" and do not cause outbreaks.
He added: "If the government ignore their own science, we have the mother of all legal cases against them. If Cinderella couldn't open, we'd go, 'Look, either we go to law about it or you'll have to compensate us.'"
This is not the first time Lord Lloyd-Webber, 73, has criticised those calling for a delay in reopening.
Last week he told the Daily Mail he may take legal action if his theatres are not allowed to welcome back crowds at full capacity.