David Cameron has been criticised after mistakenly saying the UK was the "junior partner" in the allied World War II 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.
Labour's David Miliband called it a "slight", while a veterans' group said it could "alienate" former troops.
No 10 said Mr Cameron had not meant to belittle the efforts of British troops.
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.
"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 Harbor attacks which drew the US into conflict with 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.
Mr Miliband, the shadow foreign secretary, said the prime minister's comments had been misguided.
He said: "1940 was our finest hour. 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."
Terry Burton, president of the Association for Veterans of Foreign Wars, said Mr Cameron's comments were "rather inappropriate".
"It is going to alienate a lot of veterans. He should consider his words more carefully. The UK had been fighting the war a long time before Pearl Harbor," he said.
BBC political correspondent Vicki Young said the "pretty controversial comments" were a reflection of Mr Cameron's strategy to ease the fixation on the concept of the special relationship between the UK and US.
"He was trying to be realistic about Britain's position in the world, but it does seem to have backfired in this case," she said.
Before leaving Washington DC for New York, Mr Cameron laid a wreath at the US military's Arlington National Cemetery in Virginia.