Pizza Express is to close 73 of its UK restaurants with the potential loss of 1,100 jobs.
The chain, which at the moment has 454 UK outlets, said it had cut a deal to reduce rent costs.
It said although most of its restaurants have been profitable over the past three years, earnings had been declining.
Pizza Express also confirmed it had hired advisers from Lazard to lead a sale process for the business.
It is currently majority owned by Chinese firm Hony Capital.
Zoe Bowley, Pizza Express's managing director for the UK and Ireland, said: "Unfortunately, the impact of the global pandemic has meant that we have had to make some incredibly tough decisions to safeguard Pizza Express for the long term."
Ms Bowley said that in most cases, the stores selected for closure are near to another Pizza Express that has already reopened or will be reopening soon.
She added that the process "will protect the jobs of over 9,000 of our colleagues".
The outlets to close are:
Aberdeen, Belmont St
Barnstaple, Three Tuns
Birmingham, Corporation St
Bournemouth, Post Office Road
Bristol, Berkeley Square
Bristol, Regent Street
Bruton Place, London
Charlotte Street, London
Dudley, Merry Hill
Earls Court, Earls Court Road, London
Fulham Palace Road, London
Glasgow, Princes Square
Ipswich, Lloyds Avenue
Leeds, Crown Street
Milton Keynes, Hub
Newport, Isle of Wight
Newport, South Wales
O2 Finchley, London
Oxford, Oxford Castle
Reading, St Mary's Butts
Sheffield, Devonshire Street
Sheffield, The Moor
Southport, Old Bank
Wardour Street, London
A brief history of Pizza Express
1965: Pizza Express founder, the late Peter Boizot, brought a pizza oven from Napoli and a chef from Sicily to open his first restaurant in London's Soho.
1992: Mr Boizot grew his empire over the following almost-three decades before selling it for £15m to Hugh Osmond and Luke Johnson, the man who was - until recently - chairman of Patisserie Valerie. They floated it on the stock market the next year and ultimately sold out in 1997 when it was worth £150m.
2003: It was taken private again in a £278m deal by two private equity firms who then floated it two years later - although it lasted less than a year on the public markets before it was returned to private equity hands.
2014: It changed hands again, this time to be acquired for £900m by its current owner, Chinese private equity house Hony Capital.
2020: It has more than 600 restaurants globally: 454 in the UK, including five franchises; 19 in Ireland; 24 in Hong Kong; 6 in Singapore; 14 in UAE; 60 in China; and 49 other international sites operated by franchisees.
The government has been running its Eat Out to Help Out scheme in August to try to help revive the flagging UK hospitality sector.
Diners used the Eat Out to Help Out scheme more than 35 million times in its first two weeks.
Pizza Express has been taking part in the scheme, and has been reopening restaurants that had been temporarily closed to participate.