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.
More top news
This weekend will be mostly dry with sunshine on Saturday and cloudy, windy conditions by Sunday.
A rare bird's making a comeback to Norfolk and Suffolk. The Stone Curlew is now thriving in the Brecks after falling numbers.
Tributes have been made to a Chelmsford teenager who died in hospital after he was stabbed in Oaklands Park.