Bristol Zoo has warned its future is "seriously at stake" because government law dictates it must remain closed as the pandemic continues.
A recent announcement means the zoo, and Wild Place Project, must keep their gates shut despite the phased re-opening of other public spaces.
Some non-essential shops are preparing to re-open from 15 June and other public places, like pubs and cinemas, will follow from July.
A spokesperson from Bristol Zoological Society says the imposed closures have "sparked outrage" from zoos across the country and mean they may never recover from the effects of recent months.
On Thursday (11 June) MPs will discuss the issue in a parliamentary debate following a request from the British and Irish Association of Zoos and Aquariums.
We have now reached a point where the future of zoos, including Bristol Zoo Gardens, is seriously at stake. Our staff, volunteers, visitors and supporters are outraged and perplexed at this decision.
At the start of the pandemic the society revealed it would lose around £800,000 a month between Bristol Zoo and Wild Place Project for every month it was to remain closed.
A fundraising page was launched to help cover some of the costs and has raised more than £27,000 so far.
Only key members of staff like vets and keepers have continued to work, while most employees were furloughed.
Dr Morris added: “Like any responsible charity we had financial reserves in place to mitigate against the immediate effects of being closed and have made the most of the government measures to offset costs.
"There is only so much we can do. We have a living collection of animals to continue to care for and all of the costs associated with that. We cannot furlough all of our staff and we must also ensure that our sites are secure and maintained.
“The reality is that the cash has now run out and the longer we remain closed to the public, the greater the challenges we will face returning to full capacity in the future."
Bristol Zoological Society's predicted income in July and August
The zoo's bosses are concerned they could be forced to remain closed for a while longer with the peak summer months fast approaching.
Dr Morris described the idea of this as "unthinkable" and said it "makes absolutely no sense" to keep the attractions closed when other outdoor spaces are allowed to re-open with measures in place.
He said staff members have been "working tirelessly" to implement health and safety measures to keep visitors socially distanced.