The Covid-19 lockdown has cost the UK's most visited zoo £5m and put it "at risk of extinction", its boss has said.
Jamie Christon said Chester Zoo, which has been closed to visitors since 21 March, had been told by the government to prepare to stay shut "indefinitely".
He said being unable to open meant it faced a year-end debt of more than £24m that "will financially cripple us".
The government said it had provided a £14m fund to zoos to support animal care.
A Department for Environment, Food and Rural Affairs (Defra) spokesman added that while zoos had not been included in the list of outdoor ticketed venues that can reopen, work "to understand how and when" they may be able to was "ongoing".
Mr Christon, the zoo's chief operating officer, said not being included on the list "flicked a switch for us and, heartbreakingly, our lights are now flickering".
"Not being able to open, despite being a huge outdoor site, is having a devastating impact.
"We're heading towards debt in excess of £24m by the end of 2020 [and] this will financially cripple us."
'Huge financial cost'
Mr Christon said the zoo, which welcomed two million people in 2019, normally gets 97% of its income from visitors and needed more than £1.6m a month to cover the cost of its staff, 128-acre site, conservation and breeding projects and 35,000 animals.
The zoo has been running virtual tours on Facebook in efforts to engage audiences and previously announced it was finalising plans to reopen with special measures designed to maintain social distancing.
Mr Christon said he and his colleagues "absolutely refuse to cut corners when it comes to caring for the animals", but their care "comes at a huge financial cost".
"Not being able to open, with such massive outgoings, puts the future of the zoo itself at risk of extinction."
Ellesmere Port and Neston MP Justin Madders said the situation was "incredibly concerning".
The Labour MP, in a joint letter with party colleagues Chris Matheson, who represents Chester, and Weaver Vale's Mike Amesbury, has called for the government to take action to save the zoo.
The news we have heard from @chesterzoo today is incredibly concerning- myself @ChrisM4Chester & @MikeAmesburyMP have therefore written a joint letter to the Government asking them to take action to save the zoo. pic.twitter.com/RPlOGLcJt2— Justin Madders MP (@justinmadders) June 3, 2020
Local Conservative MPs - Eddisbury's Edward Timpson, Warrington South's Andy Carter, Tatton's Esther McVey, Crewe and Nantwich's Kieran Mullen and Wrexham's Sarah Atherton - have also spoken out in support of the zoo.
Mr Carter said they were "working hard to highlight" the zoo's "raft of safety measures" to ministers and lobbying "for the reopening of our zoo as soon as possible".
Ms McVey added that she was "concerned that a blanket ruling for zoos to remain closed doesn't reflect the ability for zoos as large as Chester to manage social distancing adequately".
The Defra spokesman said the government "understand the challenges" faced by the zoo "during these unprecedented times, but it's vital that we do not move too quickly in reopening to ensure public health is protected".
He said Defra had "provided a £14m support fund to ensure zoos are able to continue to care for their animals" and "work is ongoing to understand how and when [they] may be able to reopen in a safe way to the public whilst maintaining social distancing".