Papers published today under the 30-year rule reveal that Government officials considered plans to flood Essex if the capital was threatened by a tidal surge.
A Home Affairs department paper later points out that flooding Essex to protect London "seems unlikely to prove practicable".
Large sections of defences would have to be destroyed "which together have cost £250 million".
Explosive charges would have to be laid in advance but there were doubts about when exactly to trigger them.
It was noted:
The last measurement of tidal surge is at Southend, approximately an hour before it would reach central London, but the height of the eventual tidal wave in central London could not be predicted with complete accuracy, and thus the defences could well be blown up unnecessarily.
There is a major political difficulty in that the Government would have to take responsibility for deliberately flooding Kent and Essex in order to protect central London.
It seems doubtful therefore whether the breaching of the downstream walls should be contemplated. If the committee decided that the possibility was worth considering, it would need not only very thorough investigation but also most sensitive handling because of the alarm which might be created in Canvey Island and elsewhere"
Government officials considered deliberately flooding parts of Essex to stop central London being swamped by a tidal surge.
The cost, the need for explosive charges to be laid in advance and the potential legal wrangles which could follow were all problems.
Either way it "needs thorough investigation" and "sensitive handling" but it is "doubtful whether this option should be contemplated", it was noted.
The document states a severe London flood would be "the most damaging natural disaster liable to affect these islands"
There were fears of looting, civil disorder and "mass evacuation may be needed" as large parts of the flood zone would be virtually uninhabitable.
The document states:
It is generally accepted that there would be casualties, resulting from such causes as collapse of buildings, open manholes in flooded roads and individual failure to heed warnings and that, depending on the severity of the flood, deaths could be numbered in the hundreds rather than dozens.
Government officials considered deliberately flooding parts of Essex to stop central London from being swamped by flood waters, according to newly-released papers.
It was suggested as gates for the Thames Barrier were stranded by a dock strike in Teesside in the late 1970s.
The idea was to breach flood defences downstream so that some of the water would flood in to low lying land in Essex and Kent, the contingency planning documents dating from July 25, 1979 to December 22, 1983 from the National Archives state.
Such drastic action would have meant "major political difficulty" for the Government, it was stated.