The London-listed group, which is also behind the Mecca and Enracha brands, said it slid to a post-tax loss of £72 million in the six months to end June, down from a £9.4 million profit a year earlier.
Net gaming revenues fell by 48% to £329.6 million.
Nearly 80% of the group's revenue is traditionally derived from from its sites. The group had also seen its UK online business face a "challenging" period, after the Gambling Commission issued new affordability guidance to protect online gamers.
Rank's chief executive, John O'Reilly, told the Standard the group was losing £15 million per month while shuttered, and that London - a key driver of revenues for the firm's casino business - is still "a struggle" with revenues still down 40% on pre-pandemic levels today. Of Grosvenor's 52 UK sites, the nine London venues usually account for 42% of the brand's revenues.
But O'Reilly said that outside of London and Scotland revenues are just about back to pre-pandemic levels, and that the anticipated return of London office workers in greater numbers in September is likely to boost sales.
He said: "It's been a rotten year... But we've navigated our way through the pandemic well. Since opening in May we're really pleased with the progress we're making in all three businesses.
“We've got a strong balance sheet now as we re-emerge and we've been using the downtime to ensure that we do emerge from this strongly. Recovery is coming."
The CEO said the firm has been making "a good bit of investment where we can across the estate including new products and services for customers".
Shares fell by 2.4% on Thursday morning.