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.
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Morning's likely to see plenty of cloud bringing some mainly light and patchy rain.
Any showers should die away this evening with most places dry overnight.
The veteran rockers Status Quo have been forced to postpone a concert in Norwich this weekend after frontman Francis Rossi became ill