The two highly decorated panels feature large, colourful flowers, geometric patterns and elaborate motifs in a style unique to the capital.
It is thought it once decorated the floor of a Roman dining room.
The Museum of London Archaeology (MOLA) find came during excavations as part of the construction of a regeneration project near the Shard in Southwark.
MOLA site supervisor, Antonietta Lerz, said: “This is a once-in-a-lifetime find in London. It has been a privilege to work on such a large site where the Roman archaeology is largely undisturbed by later activity – when the first flashes of colour started to emerge through the soil everyone on site was very excited.”
It is made up of two panels, with the largest showing large, colourful flowers surrounded by bands of intertwining strands – a motif known as a guilloche.
There are also lotus flowers and several different geometric elements, including a pattern known as Solomon’s knot, which is made of two interlaced loops.
The mosaics will be carefully recorded and assessed by an expert team of conservators before being transported off-site, to enable more detailed conservation work to take place. Future plans for the public display of the mosaics are currently being determined.