Archaeologists have discovered the remains of a 700-year-old stone wall beneath the Palace of Westminster.
Experts believe the structure to be the original Thames River wall, which ran under the Houses of Parliament.
The remains were unearthed during work to help restore the crumbling building, which is being assessed as part of a multibillion pound restoration scheme.
It comes after medieval timber structures, also thought to be part of a river defence system, were discovered during an excavation of Black Rod's Garden in 2015.
Previous ground investigations over the past few decades have uncovered an array of historical artefacts, including a centuries-old sword and buried fragments of King Henry III's high table.
The latest discovery came during a borehole investigation in Chancellor's Court, near the House of Lords.
Over the summer and early autumn, specialists spent 4,850 hours examining 160 rooms and drilling boreholes up to 70 metres deep to assess ground conditions around the palace.
After the discovery of the remains, drilling was paused and the structure was assessed by archaeologists from the Museum of London Archaeology (Mola).
Experts removed a small amount of material for analysis before the site was sealed up to protect the structure.
They now believe the remains to be at least 700 years old and made from Kentish ragstone, a hard grey limestone quarried from Kent and used in the construction of the Tower of London and Westminster Abbey.
Roland Tillyer, archaeologist with the museum, said: "We were expecting it might be present in this area and the borehole in Chancellor's Court may have encountered it.
"The first few meters of the borehole sequence was as expected, post medieval dump deposits, which are quite soft, but then around 3.5 metres (12ft) we came across much harder material, including Kentish ragstone, mixed with a sandy mortar."
The boreholes are part of an extensive programme of building investigations by the Houses of Parliament Restoration and Renewal Delivery Authority.
Earlier this year, a report by the authority suggested the project could cost up to £22bn and take up to 76 years in order to revive the Palace of Westminster to its full glory.
They made an agreement to preserve the palace, which was rebuilt in 1876 following a devastating fire, and to seek independent advice and assurance on the new approach to the works.
Speaking of the latest discovery, Sir Lindsay Hoyle, Speaker of the House of Commons, said: "The Palace of Westminster is a treasure trove of history, and making sure this is properly conserved whilst also getting on with the vital job of restoring this unique place is a key priority."