Boris Johnson is being urged to end a lockdown block on residential school visits or risk destroying the "great British tradition" of outdoor education.
Schools have reopened, as have hotels, but official guidance still advises against overnight educational trips.
Outdoor learning "faces an existential threat", providers have told the Prime Minister in a letter.
The rules are under review, governments in England, Wales and Scotland say.
But according to the letter from UK Outdoors, which represents 15,000 people and organisations, the continuing freeze on residential school trips could cost almost 6,000 jobs before January.
The letter adds: "We cannot warn the government in strong enough terms that any decision to prevent residential trips for the rest of the academic year, without support, will permanently close the whole sector."
PGL, perhaps the best known company in the business, has announced 670 job losses, a quarter of its workforce.
Its Marchants Hill centre in Surrey would normally be buzzing with the excitement of more than 750 school children and their teachers - but last Friday, as on every day since the start of the lockdown, it was eerily silent.
Most of the 160 staff have been furloughed, leaving managers Kris Shipway and Kristina Timms to keep the site ticking over, ready to reopen.
"The sadness, that's the biggest thing," says Kris.
"It's about the experiences we're able to give to children... and not to be able to do that has been really hard."
It's a similar story at Rhos y Gwaliau, in Snowdonia, North Wales.
"We've been completely empty for six months now. We've had no children at the site and we've really missed having them here," says instructor Eve Shrimpton.
Rhos y Gwaliau has been welcoming children to the mountains from schools in and around Reading, for 40 years.
"It's something the children look forward to from the beginning of their school careers," says Eve, and for some, "it might be the only opportunity they have in their lifetime to do something like this".
The guidelines allow schools to run daytrips - but Rhos y Gwaliau is just too remote for this to work, says Eve's boss Sara Jones.
Even for bigger operations like PGL, day trips cannot compensate for the loss of their core business.
"Without overnight accommodation, the sector is effectively shut," says PGL chief executive Anthony Jones.
"For us, about 97% of our business relies on overnight accommodation."
About two million visits have already been lost, says Penrith-based Andy Robinson, chief executive of the Institute of Outdoor Learning and one of the signatories of the letter.
Many have been cancelled completely, with others postponed, but without a reopening date, the new dates might not hold.
And the loss is not just of the activities, whether rock climbing, wild camping or the opportunity to ride a zip-wire, it is the chance to be outside for hours in the middle of a wood and to stay away from home without your parents, perhaps for the first time - experiences that help build the soft-skills and resilience so prized by employers.
"It's not having that building of self confidence at key moments, for example the transition from primary to secondary," says Mr Robinson.
Or perhaps "losing a moment that can spark a lifetime interest in a particular outdoor pursuit".
Health and safety is essential to a sector which specialises in taking young people into challenging environments, so all members have signed up to extensive Covid-safe policies and procedures to keep school groups in their separate "bubbles".
These "make it quite clear that bubbles will not come in contact with other bubbles", says Anthony Jones.
"We will put in place segregation, one way systems, staggered meal times separate activities, separated accommodation."
He hopes these measures will convince both parents and policy makers that outdoor education trips are safe.
The sector feels unfairly penalised, he says: "We've answered every question that has been put to us."
"It makes no sense."
In Snowdonia, Sara Jones wants government to engage fully with the sector's plans for Covid-safe operating and to set a date for reopening.
She fears that once furlough ends this month, and without a firm reopening date, she will be unable to keep her highly skilled staff who are often qualified teachers as well as mountain activity specialists.
"We also ask that while outdoor education centres face this enforced closure that we are appropriately and adequately funded in a targeted way to ensure that we survive this crisis and are here for generations of children in the future," she says.
Anthony Jones says the sector is ideally placed to help children and young people recover from the lockdown. .
"We've got these amazing assets, we've got amazing staff that are desperate to show what they can do and how they can help."
In a statement, England's Department for Education said: "Since the start of term, schools have been able to run non-residential trips.
"We keep our guidance on both residential and non-residential trips under review, in line with Public Health England advice."
The Welsh government said it was also keeping the issue "under review", with non-overnight visits allowed, subject to thorough risk assessments.
The Scottish government said it was not possible to set out an exact date for a review of the guidance, as this would depend on the trajectory of virus transmission in Scotland between now and the new year.