Sir Keir Starmer has hailed the "closeness" of the UK and Germany, as the two nations signed a treaty that is set to free up school exchange visits and passport e-gates.
The Prime Minister and German chancellor Friedrich Merz signed the deal at the V&A Museum in London on Thursday, as Sir Keir said they will look to "work ever more closely" on issues such as trade, security and defence.
The agreement, to be known as the Kensington Treaty, was also signed by Foreign Secretary David Lammy and his German counterpart Johann Wadephul.
Speaking at the ceremony, Sir Keir told Merz: "It's a privilege to have you here today, particularly to sign this Kensington Treaty, which is a very special treaty, because it's the first of its kind ever, if you can believe it, between our two countries."
Sir Keir described it as "evidence of the closeness of our relationship as it stands today", as well as a "statement of intent, a statement of our ambition to work ever more closely together".
The document details the UK and German agreement to “reinforce Euro-Atlantic security and ensure effective deterrence against potential aggressors” through their defence forces, as well as looking to improving defence cooperation in the future.
Berlin has also agreed to allow some arriving UK passengers to use passport e-gates, initially for frequent travellers, with the system due to be in place by the end of August.
The treaty includes the UK and Germany agreeing to establish a taskforce aimed at paving the way for direct train services between the countries. It is hoped services could begin within the next decade.
The treaty also says that both nations "value bilateral school and youth exchanges" and will work to facilitate them.
Elsewhere in the treaty it says that the UK and Germany will: Cooperate to fight against organised cross-border crime in an effort to end migrant smuggling, work towards more bilateral work on defence, and pursue “deep exchanges” on security issues such as arms policy.
The treaty comes as part of a wider visit by Merz as Downing Street looks to boost ties on defence and tackling people smuggling.
Germany committed last year to make facilitating the smuggling of migrants to the UK a criminal offence.
Merz is expected to commit to adopting this law change by the end of the year. Downing Street has described the move as a "significant step".
Merz said he had been "surprised" to learn the agreement was the first UK-Germany treaty since the Second World War.
"We had you in the European Union and we thought that was enough," he said, "but we are now learning that it's not enough so we have to do more on that."
(PA/London) Note: This article has been edited with the help of Artificial Intelligence.
