Plans to stabilise the ancient Silbury Hill mound in Wiltshire have been unveiled by English Heritage.
Silbury Hill dates back to the Neolithic period
The man-made monument, believed to date to the Neolithic period, developed a hole at the top five years ago after the collapse of infilling in a shaft.
There are proposals to remove an inadequate backfill from this and other cavities and replace it with chalk.
English Heritage said it would preserve the long-term stability of the hill while minimising further damage.
Surveys have confirmed that the overall structure is stable, although there are pockets of instability resulting from tunnels dug in 1776, 1849 and 1968.
English Heritage is drawing up a brief for contractors to come forward with their proposals for how the work should be done.
The organisation is also looking at how to fund the project.
Regional director Bob Bewley said: "If all goes to plan we're probably looking at some small amount of work during 2006 and then it'll probably happen in the summer of 2007.
"But that's all dependent on what we find when we do further analysis and investigations, when we talk to the possible contractors and the big 64,000-dollar question is finding the money."