Sir Keir Starmer will have his first face-to-face meeting with US President Joe Biden in the White House later.
The prime minister travelled to the US overnight for a Nato summit, on the 75th anniversary of the North Atlantic Treaty Organisation.
Speaking on the plane, Sir Keir told reporters he hopes the bilateral in Washington DC will be a chance to talk about Nato and the so-called UK-US special relationship.
“This is obviously a very special relationship we have between the UK and the US,” he said, adding there was “a special aspect when it comes to defence and security for obvious reasons, including our commitment to Nato”.
The Labour leader was speaking as more than 300 new MPs continued to be sworn in at Westminster, ready for the state opening of parliament next week, when the government’s programme is laid out in the King’s Speech.
There were also questions for Sir Keir about whether he believed age restrictions the government plans to introduce in the House of Lords, forcing peers to retire at the age of 80, should be applied across the board.
The size and role of the House of Lords have come under increased scrutiny in recent years, with 784 sitting members, and Sir Keir has previously suggesting an alternative, elected second chamber based on models elsewhere in the world.
This would affect Labour peers such as figures like Labour’s Lord Alf Dubs, who came to the UK as a child fleeing the Nazis and has long campaigned on refugee rights, and is 91-years-old.
Sir Keir said: “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.”