Each henge consists of circular earth banks and ditches

Tarmac has applied to quarry sand and gravel at Ladybridge Farm in North Yorkshire, near Thornborough Henges.

If approved, campaigners fear the work may help destroy clues about why the 5,000-year-old earthworks were built.

The company said it believed the land could accommodate both quarrying and conservation of the henges, near Ripon.

Tarmac is continuing to play an active and supportive part in consultation on the future conservation of the monuments and adjoining area Tarmac spokesman

Local campaign group Timewatch, which has collected a petition of more than 1,500 signatures against the plans, said the quarry would contribute to the permanent loss of nationally important archaeology.

US-based conservation group the Landmarks Foundation has also weighed into the debate, describing the quarry proposals as a tragedy.

The henges are believed to represent one of Britain's largest ritual gathering places from the Neolithic period.

Tarmac already has a quarry at Nosterfield, close to the ancient henges which consist of three earthworks built in a line running north-south for about a mile.

The henges are in open countryside near the A1

Tarmac has said the planned quarry extension is on farmland where there is only "thin and scattered" evidence of prehistoric activity, according to a recent study by archaeological consultants.

In a statement, the company said the actual henges were under no threat from quarrying because of their status as protected ancient monuments.

"Tarmac is continuing to play an active and supportive part in consultation on the future conservation of the monuments and adjoining area," a spokesman said.