Britain drew up plans to cut the flow of the River Nile to Egypt to force President Gamal Abdel Nasser to give up the Suez Canal in 1956, files reveal.
Britain wanted to control the key trade route
Military officials believed they could harm agriculture and cut communications by reducing the flow of water, newly-released documents show.
The plan was outlined to Prime Minister Anthony Eden six weeks before British and French forces invaded Egypt.
But it was abandoned because of fears it would trigger a violent backlash.
Under the plan, Britain would have used a dam in Uganda to reduce water levels in the White Nile by seven-eighths.
But planners realised that the scheme would take months to work, and could also harm other states such as Kenya and Uganda.
One British official noted that the plan, while unworkable, could still be useful.
"It might be possible to spread the word among the more illiterate Egyptians that 'unless Nasser climbs down, Britain will cut off the Nile'," Cabinet official John Hunt was revealed to have said.
'No legal justification'
The Suez crisis was triggered in July 1956 when the Egyptian president nationalised the Suez Canal, a vital trading route from the Mediterranean to the Red Sea.
Britain and France joined forces with Israel and the three nations attacked in October 1956 in a bid to regain control of the canal, but US and UN pressure forced a withdrawal.
The documents, released to the National Archives in Kew, also show the prime minister was urged to conceal the fact that his attorney-general had warned that the invasion was illegal.
At the time, UK lawmakers were claiming that the action was legal.
But Attorney General Sir Reginald Manningham-Buller - father of current MI5 head Dame Eliza Manningham-Buller - wrote a strong letter challenging this.
"I am unable to devise any argument which could purport to justify in international law either our demand that she [Egypt]... should withdraw her forces from a part of her own territory which she is engaged in defending, or the threat to occupy her territory by armed forces should she fail to accede that demand," he wrote.
Then-Cabinet Secretary Sir Norman Brooks told the prime minister that he should not raise the issue of the war's legality in future speeches.
The Suez crisis damaged Sir Anthony's reputation and led to his resignation in 1957.