The group - which is being bought by US casino giant Caesars Group in a £2.9bn deal - saw underlying pre-tax profits slump to £9.1m in 2020 from £96.5m in 2019.
Its 1,414 betting shops were hit hard by repeated lockdowns and restrictions, with the retail arm slumping to a £29.5m loss as like-for-like revenues plunged 30 per cent.
William Hill's online business - accounting for 61 per cent of group revenues - delivered a 3 per cent rise in earnings to £121.9m, but this was not enough to offset the retail woes.
Ulrik Bengtsson, CEO of William Hill, said: "In what was an extraordinary year I am immensely proud of how the group has responded and the resilience we have seen in our performance.
"We prioritised the protection and safety of both our colleagues and our customers, and our employees went above and beyond for which I thank them.
"In 2020 we put our strategic plans firmly into action, diversifying our geographical footprint, expanding our team's capabilities and rebuilding our technology.
"We are embedding proprietary components across the platform architecture and are delivering a constant flow of new features including faster product experience, improved navigation and greater protection to our customers around the world.
"The performance in the second half is clear testimony that our strategy is bearing fruit. In the UK, the competitive position of our online offerings for both gaming and sports has been materially strengthened, and our omni-channel product is delivering encouraging early results.
"Retail has undergone regional disruption although where stores did re-open, they quickly traded towards pre-Covid levels.
"We are delighted with our international online performance, where our investment in our product and technology is producing clear benefits, particularly in light of the regulatory headwinds in Germany and temporary restrictions elsewhere.
"We will continue to benefit from our agile marketing engine, and the recent agreement to acquire Alfabet S.A.S. in Colombia and our licence in Argentina both offer further promising growth opportunities in Latin America.
"The US traded well into the year-end, concluding the year with 19 per cent market share and delivering a profitable return. Our partnerships have ensured that brand awareness has risen, our product offering has expanded, and our end-to-end proprietary tech is facilitating rapid new state openings.
"As William Hill embarks on a new chapter, we will continue to prioritise the protection of our customers."
William Hill employs 1,300 people in Leeds.
David Kimberley, analyst at Freetrade, said: "William Shareholders won’t love today’s results but there’s plenty to be upbeat about too.
"Shrinking revenues were largely due to Covid but they’re also a reflection of the increasingly tough regulatory and competitive market in the UK.
"Credit card bans and stake limits are not life-threatening for the group but they’re going to leave a dent in earnings. An upcoming review of the UK’s gambling laws could also mean regulation in William Hill’s home market gets even tougher.
"That’s why most shareholders will be looking to the US as their salvation. It may represent a small chunk of earnings now but that could change easily."
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