Heavy rain has forced experts to abandon a repair works on Silbury Hill.
The team of engineers and archaeologists have been working to make the landmark, the world's largest man-made prehistoric mound, safer.
But English Heritage has now stopped works because of complications caused by instability above the central chamber within the centre of the Hill.
These problems have been exacerbated by recent heavy rain.
An English Heritage spokesman for said: "Our expert archaeologists and engineers are developing a solution to take the project forward, with the aim to stabilise the hill for the long term.
"We will issue a new programme for the completion of the works as soon as possible."
Archaeologists found traces of a Roman settlement at the landmark in the spring.
English Heritage believes there was a Roman community at Silbury Hill about 2,000 years ago. It said the site may have been a sacred place of pilgrimage.
The 130ft Neolithic mound near Avebury is thought to have been created some 3,000 years earlier.
Parts of the ancient site are thought to be collapsing because of the tunnels dug by archaeologists over many centuries.
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