A 14th Century moat which was covered with rubbish in the 1920s is being excavated at Fulham Palace.
The work is part of an £8m improvement programme to the palace and adjoining Bishops Park.
The first known reference to the moat dates back to 1392, although some historians believe it may date back to the Iron Age.
Archaeological consultant Phil Emery said it was important to preserve the moat for future generations.
Mr Emery, who is leading the project on behalf of Hammersmith and Fulham Council, said "The present excavation and opening up of this key section at the main entrance is only the beginning.
"In the longer term, we would like to extend the restoration scheme to include other prominent parts of the moat circuit."
A council spokesman said the moat was filled in with builders' debris during the 1920s by Fulham Council at the request of the Bishop of London who lived in the palace.
"That move led to public outcry and the issue was even raised in the Houses of Parliament," he added
He said the moat was filled in with items such as glass and stoneware bottles, china, teapots, porcelain dolls and even a perfectly preserved 'Lipton Tea' enamel sign which has now been unearthed.
The entire moat is preserved below ground as an unbroken circuit.
Councillor Greg Smith, cabinet member for residents' services, said: "This historic moat is one of the most significant heritage assets in the borough and it is enthralling to see this work occur.
"It is absolutely fascinating to think that this moat used to encircle the palace. The moat has been the topic of much discussion over the years so it is wonderful to be able to see certain parts of it at first hand."
Over the next few months, the council will be restoring the moat bridge, Gothic Lodge, the vinery and potting sheds.
The stables will also be converted into a new education facility, with new toilets and a shop built.