Stone will continue to be quarried from Ham Hill Country Park near Yeovil, Somerset, for the next 80 years after planning permission to extend the site was agreed.
The Iron Age hill fort, near Norton sub Hamdon, is the only place in the country where ham stone can be quarried but as of July 2010 it was estimated the current reserve would only last for 18 months.
With much of the remaining stone to the south of the park being deemed unusable, a new source was needed.
Permission has now been given by Somerset County Council to extend the quarry to the east of the current site.
Output will be limited to 6,000 tonnes a year although that figure is more likely to be 3,000 tonnes.
It is estimated the new site will be produce stone for the next 80 years and that 80 per cent of the quarry output would be used locally.
None of the nearby parish or district councils had any objections to the plans.
English Heritage said the plans presented a dilemma because, while ham stone was needed to restore historical and important buildings, extending the quarry could impinge on a historical site.
Giving its view of the plan, it said: "Ham Hill is both the main source for the continuing supply of ham stone, used historically for many important buildings, and is the site of the largest hill fort in England. This presents a policy dilemma."
The county council's heritage department said the need for ham stone balanced out any possible damage to the site.
The department said: "The public benefit of the proposal should be considered alongside the loss. In this case the applicant is putting forward a mitigation strategy alongside a research and outreach project that will lead to quality information concerning the archaeological remains on the site, address significant research aims within the hill fort and engage the public in a three-year project.
"It is our view that in this case the loss/harm to the heritage asset can be mitigated by the submitted project."
The Harvey Stone company will quarry and sell the stone, which is largely used for buildings, walling, roofing and architectural products.
Many local historic buildings are constructed of ham stone, such as the National Trust's grade one listed Montacute House. It has been quarried at Ham Hill since Roman times.
The site is designated as a Scheduled Ancient Monument (SAM) and is the largest Iron Age hill fort in England.
Most of the hill is also a country park although the existing quarry and the proposed extension are excluded from this.