Sir Keir Starmer has described President Donald Trump's remarks about British troops in Afghanistan as "insulting and frankly appalling".
On Thursday, Trump said that US allies stayed "a little off the front lines" in the conflict and claimed he was not sure Nato "would be there" if the US needed it.
Starmer said if he himself had "mispoken in that way", he would "certainly apologise".
Trump's words sparked international outrage as the UK and other nations joined the US in Afghanistan after Nato's collective security clause was invoked following the 9/11 attacks. During the conflict, 457 British service personnel were killed.
"I will never forget their courage, their bravery and the sacrifice they made for their country," Starmer said.
