Sir Keir Starmer has branded Rupa Huq's comments about Kwasi Kwarteng being "superficially" black as clearly "racist".
The Labour leader welcomed the party taking "very swift action" in suspending Ms Huq's membership in the wake of her remarks at a fringe event at the Labour Party conference in Liverpool.
Ms Huq, the MP for Ealing Central and Acton in west London, offered a "sincere and heartfelt" apology to Mr Kwarteng over her "ill-judged" comments, after she was criticised by senior Labour figures.
"I have today contacted Kwasi Kwarteng to offer my sincere and heartfelt apologies for the comments I made. My comments were ill-judged, and I wholeheartedly apologise to anyone affected," she said on Tuesday.
Ms Huq was nevertheless administratively suspended from the party and lost the whip.
'What she said, in my view, was racist'
Commenting on the incident, Sir Keir told LBC Radio: "What she said, in my view, was racist, it was wrong and she's been suspended from the whip in the party and that was done very, very quickly.
"I think that tells you how strongly I feel about those comments."
The Labour leader said it was not his decision to suspend Ms Huq, but welcomed it as one "I absolutely agree with", saying that the MP now faces an investigation.
Sir Keir also conceded it was "frustrating" to have to discuss the comments during the party's conference that has otherwise been seen as a success.
'Superficially he is a black man'
In audio published by the Guido Fawkes website, Ms Huq was heard discussing Mr Kwarteng's elite school background, before saying that "you wouldn't know he is black" when listening to him on the radio.
"Superficially he is a black man," she told the event. "He went to Eton, I think. He went to a very expensive prep school. All the way through, the top schools in the country.
"If you hear him on the Today programme, you wouldn't know he is black."
Jake Berry, the Tory Party chairman, raised his "serious concerns" in a letter to Sir Keir and demanded Ms Huq's suspension.
