David Cameron has slipped up by mistakenly saying that the UK was the "junior partner" in the allied fight against Germany in 1940.He made the historical slip, neglecting the fact that the US had yet to enter the war, on the second day of his first trip to the US as prime minister.
No 10 said Mr Cameron had not meant to belittle the efforts of British troops.
But shadow foreign secretary David Miliband said it was a "slight" on the memory of their sacrifices.
Mr Cameron referred to the situation in 1940 during an interview with Sky News in which he was asked about the changing nature of the "special relationship" with the US and his meeting with President Obama on Tuesday.
'High regard'"I think it is important in life to speak as it is and the fact is that we are a very effective partner of the US but we are the junior partner," he said.
"We were the junior partner in 1940 when we were fighting the Nazis."
However, the US officially declared war on Germany on 11 December 1941, shortly after Hitler launched hostilities against the US and four days after the Pearl Harbour attacks which drew the US into conflict against Japan.
The US had been supplying the UK with war materials for the previous nine months.
Asked about his remarks, No 10 said Mr Cameron had been referring to the "current relationship between the UK and the US."
"He holds the armed forces in a very high regard," a spokeswoman said.
'Wrong'Mr Miliband said Mr Cameron's comments had been misguided.
"1940 was our finest hour," he said. "Millions of Britons stood up and gave their lives to defeat fascism.
"We were not a junior partner. We stood alone against the Nazis. How can a British prime minister who bangs on about British history get that so wrong?
"It is a slight, not a slip."
Before leaving Washington DC for New York, Mr Cameron laid a wreath at the US military's Arlington National Cemetery in Virginia.
Mr Cameron has been meeting business and financial leaders in New York as he is seeks to drive increased investment in the UK.
He will later holds talks with the UN Secretary General Ban-Ki Moon.