The Great Medieval Fire In Southwark.

1212 July 10th. The Great Medieval Fire in Southwark, The second of the two great medieval fires of
London, also known as "the Great Fire of Suthwark" began in Southwark, the borough directly to the south of London Bridge. The flames destroyed Our Lady of the Canons (Southwark Cathedral, also known as St Mary Overie) and strong southerly winds pushed them towards the bridge, which also caught fire. London Bridge had only just been rebuilt in stone, and the structure itself survived the blaze. However, King John had authorised the construction of houses on the bridge, the rents from which were supposed to pay for its maintenance, and it appears that these were lost to the flames.
The earliest account of the blaze appears in the Liber de Antiquis Legibus("Book on Ancient Laws"), composed in 1274 and today the oldest book preserved among the records of the City of London Corporation. This states: "In this year was the Great Fire of Suthwark, and it burned the church of St Mary, as also the Bridge, with the chapel there, and the great part of the city." According to later traditions, however, numerous casualties were incurred when a mass of citizens from London rushed onto the bridge at the first signs of fire, intending to cross the river to help extinguish the flames. High winds carried red-hot embers across the river and ignited buildings on the north side of the structure. This fire trapped a large number of people, many of whom died either in the blaze or while attempting to escape on overloaded boats that had come to their aid. One later chronicle related: "An exceeding great multitude of people passing the Bridge, either to extinguish or quench it, or else to gaze at and behold it, suddenly the north part, by blowing of the south wind, was also set on fire, and the people which were even now passing the Bridge, perceiving the same, would have returned, but were stopped by the fire."
Some estimates put the number of people killed on London 
Bridge alone at 3,000, but although this figure still appears in the Guinness Book of Records, it is not contemporary and is certainly an exaggeration; the total population of the whole city at this time was no more than 40,000 to 50,000 people. No reliable evidence survives to allow an accurate estimate of the number of casualties caused by the great fire of 1212, but it is known that the damage done to London Bridge was such

that the structure remained a ruin, only partially usable, for years afterwards.the flames destroyed Southwark Cathedral and the strong southerly winds pushed the flames towards the newly completed London Bridge, which also caught fire. King John had authorised the construction of houses on the bridge, and it appears that the buildings on London Bridge created a major fire hazard and served to increase the load on its arches, both of which may have contributed to the several disasters on the bridge.