The remains of a major Roman bath house have been found near the Thames in Southwark on land being cleared by Network Rail for the Thameslink project.

Archaeologists are still working on the site, but it appears to be a complex including a cold plunge bath and rooms heated by underfloor hypocausts, and is one of the most significant finds on the South Bank in recent years. The site is on the corner of Borough High Street and London Bridge Street – the main road to the oldest bridge crossing the Thames.

Finds in recent decades including cemeteries and foundations of buildings have transformed archaeologists' understanding of life on the opposite bank from the walled city of Londinium.

Chris Place, an archaeologist for Network Rail, said: "This is a significant find and offers a further insight into London's long history. In Roman times the main settlement was on the north bank of the river Thames and was connected to the settlement at Southwark by the first London bridge. Much archaeological work has been done in Southwark over the years, but we were still surprised to discover ruins of this nature and size."

Southwark, a marshy area of small islands separated by creeks, was a major settlement, with important civic buildings, and the claimed grave of a female gladiator – a unique find if true, though hotly debated.

Many finds suggest much of the area was, as in Shakespeare's time, a rackety district where people went for entertainment to brothels, taverns and games.

The bath house site has been earmarked for a new office block, but the developers are exploring how to preserve the remains, and will give key finds for display to the Museum of London.