The shocking idea was a sparked by work on the Thames Barrier being stalled by strikes in Teesside, sparking fears the capital would be defenceless in a flood.
The Government believed weakened flood defences downstream would allow some water to affect low lying areas instead of the densely populated capital.
The documents admitted that such a drastic plan would cause 'major political difficulty' for the State.
The official documents, dated from July 25 1979 to December 22 1983, were retrieved from the National Archives state.
The secret papers incredibly noted that 'sensitive handling' was needed for the issue which they admitted would result in casualties.
The building of the Thames Barrier was eventually built of time after Acas intervened and a settlement was agreed on.
However, at the time the cost of a major flood in London was seen as 'incalculable'.
The building of the Thames Barrier was held up by the dock dispute in Teesside as four gates needed to complete work on it could not be delivered.
In February 1982 then leader of the Greater London Council Ken Livingstone wrote to the Transport, Environment and Employment secretaries.
He urged them to step in and resolve the dispute to ensure that work on the flood barrier could be completed.
He wrote: "Any initiative could prove valuable in achieving the urgent release of the barrier gates. It has been suggested that a relaxation of current financial constraints might possibly be helpful in resolving the overall dispute."
Behind closed doors the Government resisted the pleas of Mr Livingstone and the dispute was settled by the non-departmental arbitrary service.
A Home Affairs department paper later said the plan to flood Kent and Essex was 'unlikely to prove practicable' and that together it would cosy £250 million.