The ensign, captured from the French warship Le Genereux on February 18, 1800, was gifted to the city of Norwich in eastern England.
Measuring 16 metres (52.5 feet) by 8.3 metres (27.2 feet), the flag will be the centrepiece of an exhibition at Norwich Castle exploring the relationship between Britain's Admiral Horatio Nelson and his home county of Norfolk.
The castle museum believes the tricolour could be one of the earliest — if not the earliest — in existence.
The blue, white and red colours — in that order from left to right — were officially adopted in 1794.
The flag will be on view at the castle from July 29th to October 1st.
Le Genereux was one of only two ships of the French fleet to escape the Battle of the Nile in 1798, the victory which sealed Nelson's reputation.
The ship was captured in 1800 and the ensign sent to Norwich, where it was put on display until 1897, before being placed in the Norwich Castle keep for the 1905 centenary of the Battle of Trafalgar.
The Norfolk Museums Service and the Costume and Textile Association are attempting to raise £40,000 ($50,000, 47,000 euros) towards its conservation and putting it on permanent display.
“The ensign is remarkable for its survival in such a complete state, given its age and inherent fragility,” said Ruth Battersby-Tooke, curator of costume and textiles at Norwich Castle.
It is “the oldest French ensign in the UK and the one with the most stirring and thrilling history”, she added.
Initial conservation efforts have discovered a nail hammered through the rope which would have been used to nail the colours to the mast.
Fragments of wood, likely to be splinters from battle-damaged ships, and traces of gunpowder have also been found.