Archaeologists working on the annual Bamburgh Research Project believe the stonework – one of a series of ancient walls unearthed by castle staff during routine maintenance work - could be the walls of St Oswald's Church.
According to history books, this was a sacred building which housed a reliquary containing the arm of St Oswald, the famous Anglo Saxon king who reigned at Bamburgh in the 7th century.
The stonework was found beneath the castle's current chapel, a Victorian folly built on the ruins of the 12th century chapel by the Lord Crewe Trust.
It shows three layers of masonry, the earliest of which experts believe almost certainly dates back to pre-Norman conquest times - further evidence they say that the stonework could be that of Anglo Saxon Church.
Paul Gething, a leading archaeologist and director of the Bamburgh Research Project, said: "This find has the potential to be massive. St Oswald's Church is one of the earliest buildings referred to in the historical records.
"The lives of saints are often shrouded in mystery, but when you encounter something tangible like this it really is exciting.
"Often people disbelieve history because it's been embellished, but seeing stonework in the ground like this, which we believe could be part of St Oswald's Church, brings history to life and makes it all very real."
Graeme Young, director of archaeology for the Bamburgh Research Project, added: "We are very excited about this find.
"It is exceptionally rare to come across a building from this period which is made of stone – usually they are wooden - which itself is another sign of the key importance of Bamburgh in Anglo Saxon times.
"Bamburgh has been an occupied site for at least 4000 years.
"The ancient evidence of its prehistoric inhabitants remains to be discovered as we dig deeper in future seasons," he added.
"This summer we have uncovered extensive evidence dating back to the early medieval period, more than 1000 years ago.
"The discovery of buildings beneath the 12th century chapel, in the inner ward, represents new and extremely exciting evidence of this most important period of the castle's occupation, a time when it was the palace-home of the kings of Northumbria."
He continued: "Normally, churches are rebuilt on the foundations of their predecessors, preserving the sacred association of the site, and it is this that has led us to believe that one of the early walls revealed beneath the 12th century church could be a part of the Anglo-Saxon predecessor that held the relics of St Oswald for hundreds of years."
Francis Watson Armstrong, owner of Bamburgh Castle, said: "Bamburgh is a treasure trove of archaeological secrets.
"It has been ranked as one of the top seven sites in the world for its archaeological importance as this most recent find supports.
"Little by little, we are finding out more about the castle's incredible history and, thanks to the hard work of experts like the Bamburgh Research Project, it is being translated so we can learn more about this remarkable site here at Bamburgh."
Chris Calvert, Bamburgh Castle administrator, said: "This is a staggering chance find which we are all hoping will help us piece together yet more of the castle's extraordinary history.
"We are enjoying our busiest year yet at Bamburgh and this latest discovery is just another reason for people to come to the castle and learn about its ancient past as well as its more recent history."
After the summer excavations are completed, pieces of animal bone and soil samples will be examined for evidence of the diet of the early inhabitants of Bamburgh, while carbon dating will help experts to determine just how old the stone structures are.
The discovery of the stonework is the latest major find to be uncovered at Bamburgh Castle this summer.
Volunteers have been carrying out excavations since June and their finds include a piece of gold gilded horse harness, a section of sword and a decorated belt buckle.
Visitors to Bamburgh Castle can watch excavations in progress, talk to archaeologists and view a display of major finds in the Archaeology Museum.
Children can experience first-hand the thrill and excitement of the dig with their own excavation area.