The social media message posted in the prime minister's name as she prepared to host international commemorations in Portsmouth, said that "Today marks 75 years since the D-Day landings".
But the date of the landings on Normandy beaches was June 6 1944, not June 5.
The prime minister was today being joined by the Queen, US President Donald Trump and 14 other world leaders to honour the memory of the 156,000 Allied troops who landed in Normandy in the largest seaborne invasion in world history, which paved the way for the liberation of continental Europe from the Nazis.
Some parachute operations took place on June 5, but the landings themselves were fixed for the following day, after being delayed 24 hours by bad weather.
The post on Ms May's Twitter feed stated: "Today marks 75 years since the D-Day landings. This was the beginning of the end of World War Two."
It was hurriedly removed after the error was spotted. A Downing Street spokesman said: "This tweet was put up in human error and was taken down very quickly."
Ms May and Mr Trump were among 16 countries agreeing a joint "D-Day proclamation" pledge to ensure the "unimaginable horror" of World War Two is not repeated.
The prime minister is set to use the occasion to call for continued Western unity in tackling what she called "new and evolving security threats".
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