Schools trips to hundreds of castles, stately homes and ancient ruins will become impossible this winter because of Government cuts.
English Heritage will shut most of its 400-plus historic buildings and monuments during weekdays between November and March, citing a 32 per cent cut in its Government grant.
The decision will mean sites such as Lindisfarne Priory in Northumberland and Deal Castle in Kent are effectively closed for school visits for five months of the year.
And English Heritage is warning schools that they need to book now to guarantee free visits to the sites over the next two summers, as the closures will make the remaining available slots "more popular than ever".
Heads' leaders are describing the closures as "deeply regrettable" and say they will hit the most disadvantaged pupils the hardest.
The quango said visits to its sites were "proving as valuable as ever for learning groups". One teacher told a recent survey: "English Heritage sites are a fantastic resource, which are well worth using."
Russell Hobby, general secretary of heads' union the NAHT, said: "Some of the richer aspects of the curriculum are being taken away with these cuts.
"We could end up with a very drab education system after a few years. It is hard to see what will be left beyond the bare bones."
But English Heritage says that only a small minority of its educational visits take place in winter.
It is keeping Kenwood House in north London, and Stonehenge and Old Sarum in Wiltshire open for school trips throughout the year.
Another 24 of its most popular sites will be available for pre-booked educational visits on winter weekdays.
A spokesman said: "Having looked at when visitors enjoy our sites, and given the 32 per cent reduction to our Government grants, we have adjusted the winter opening times.
"Inevitably, these changes will affect education visits, but with the majority of our sites being outdoor or ruined sites, the number of winter education visits is relatively low."
Association of School and College Leaders general secretary Brian Lightman said: "Some of the most inspiring learning experiences take place outside the classroom.
"At a time when the Government is emphasising history as a subject, it is deeply regrettable that these opportunities will not be available to many students, particularly those from disadvantaged backgrounds who don't have the opportunities to visit such places with their parents at weekends."
A Department for Culture, Media and Sport spokeswoman said: "Every sector is having to play its part in reducing the deficit and difficult decisions have had to be made.
"As with all our arms-length bodies, it is for English Heritage to decide how they make savings. However, we understand they are doing all they can to make sure some school trips at its busiest sites can continue through the winter months."
Original headline: Out of bounds: historic sites forced to shut to school trips