Sir Keir will not meet the Democratic presidential candidate, Kamala Harris, on this trip.
They have not met before either.
He has previously said he would like to see both candidates ahead of the US presidential election on 5 November.
The prime minister said he “obviously” still wants to speak to Vice-President Kamala Harris, but that there were the “usual diary challenges”.
He added: “I’m a great believer in personal relations on the international stage. I think it really matters that you know who your counterpart is in any given country, and know them personally, get to know them face to face.”
Harris is in Washington for talks alongside President Joe Biden with President Volodymyr Zelensky, as a row between the Ukrainian leader and the Trump campaign escalated.
Republicans have reacted angrily to the Ukrainian president’s decision to visit an arms factory in the US president’s hometown of Scranton, Pennsylvania, with several top Democrats, and it now appears Zelensky will not meet Trump as had been expected.
When asked by reporters if a Donald Trump presidency would leave the UK more exposed on the global stage when it comes to support for Ukraine, the prime minister said: “The first thing I think is important to say in relation to this is we’ve obviously had a special relationship with the US for a long time, forged in really difficult circumstances.
“That always sits above whoever holds the particular office, either in the US or the UK.”
He said the relationship was probably as “as strong now as it’s ever been” in relation to the Middle East and Ukraine”.
Starmer added: “The US people will decide who they want as their president, and we will work with whoever is resident, as you would expect. I’m not going to speculate on what any particular issues may be the other side of the election.”
