Sir Keir Starmer will meet Joe Biden to strengthen the "very special relationship" between the UK and US on Wednesday.
The new prime minister is meeting the US President for the first time at the Nato summit in Washington.
The president, 81, is seeking re-election in November despite discontent within the Democrat ranks about his suitability due to his age and concerns about his health.
Sir Keir, who has proposed forcing peers to retire from the House of Lords at 80, sidestepped a suggestion that meant Mr Biden was too old, instead insisting he was motivated by reducing the size of the Upper Chamber.
In Washington, Sir Keir will meet Mr Biden at the summit before having a one-to-one conversation in the White House.
Sir Keir told reporters accompanying him on his first international trip as Prime Minister: "I've already had a phone call with President Biden.
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"I want to follow up on that, this is obviously a very special relationship we have between the UK and the US.
"We have, within that, a special aspect when it comes to defence and security for obvious reasons including our commitment to Nato.
"We make a unique contribution in Europe to Nato and therefore it's a very good opportunity for me to talk to the President about how we take forward the important work at this summit."
Asked whether he believed his plan for peers to retire at 80 should apply across the board, Sir Keir said: "In terms of the age in the House of Lords, the simple fact is that our House of Lords is massive.
"It's the second biggest political chamber in the world. I think it's only the Chinese who have a bigger political chamber than our House of Lords, we have to reduce it.
"That is the primary driver of the retirement at 80. You can see why that needs to be done.
"We've got 800-plus members of the House of Lords, it's simply too big. We need to reduce it.
"So it doesn't reflect on how other elected representatives are chosen in other countries, it's to do with the size of the House of Lords."
