Lady Starmer had been sitting next to Ms Sarpong at the catwalk event earlier in the day.
A copy of a speech by David Pemsel, the British Fashion Council chairman, released in advance of the event, began: “Thank you, Prime Minister and Mrs Starmer for welcoming us to No 10 today. Given everything that’s going on in the world, we genuinely appreciate your hospitality and time.”
Caroline Rush, the chief executive, had planned to begin: “Prime Minister, Mrs Starmer, designers, colleagues, and friends.”
Both their comments suggested they expected Lady Starmer to attend.
On Monday, Sir Keir defended accepting gifts from Lord Alli, who was later given a pass to access Downing Street.
The Prime Minister suggested he would continue to take gifts from him despite a row over some of his donations not being declared in line with parliamentary rules.
The Labour peer has given Sir Keir tens of thousands of pounds to spend on suits and spectacles, on top of the gifts given to his wife.
The Prime Minister only declared the gifts to Lady Starmer, thought to have to be received both before and after he entered Downing Street, last week after initially believing they did not need to be added to the register.
Lady Starmer is one of a number of Prime Minister’s wives to have worn clothes by British designers to showcase the country’s fashion industry. She has also worn dresses by the British brand Me+Em.