The trip comes a week after undocumented migrants also topped the political agenda during a state visit to Britain by French President Emmanuel Macron.

UK Prime Minister Keir Starmer and Merz signed the first ever "friendship treaty" between their countries at London's Victoria and Albert Museum before heading for talks at the PM's Downing Street office.

Merz said London and Berlin had agreed an exchange programme for German and British students.

Speaking in German, he said he believed allowing the "young generation" to get to know each other and their respective countries was a "good basis for the further development of our relations".

The two leaders were also expected to unveil a deal to jointly produce military goods such as Boxer armoured vehicles and Typhoon jets, which could lead to "billions of pounds of additional defence exports," Downing Street said.

They were to commit to developing a precision strike missile with a range of more than 2,000 kilometres (1,250 miles) in the next decade.

"Chancellor Merz's commitment to make necessary changes to German law to disrupt the supply lines of the dangerous vessels which carry illegal migrants across the Channel is hugely welcome," Starmer said ahead of the talks.

His office said it was hoped the German legal changes could be made "this year".

Undocumented migration has become a major headache for Starmer's year-old Labour government, as support for the upstart anti-immigrant Reform UK party soars.

More than 22,500 would-be asylum seekers have arrived on England's southeastern coast by small boat from northern France this year alone.

The "friendship treaty" also seeks to improve post-Brexit ties with its neighbours.

- Brexit trauma -

Macron's trip in early July was the first state visit to the country by a European Union head of state since Brexit -- the UK's acrimonious 2020 departure from the bloc.

A German government source said "we shouldn't underestimate" how much relations with the UK had improved since the "traumatic" experience of Brexit.

The friendship deal would be a "foundation on which we go further to tackle shared problems", Starmer said.

The two leaders were also to discuss continued support for Ukraine, with both countries expected to play a role in US President Donald Trump's plan to send weapons to Kyiv with financing from other NATO countries.

The visit is Merz's first to the UK as chancellor, although he has already met Starmer several times, including on a trip by train to Ukraine just days after he took office in early May.

The wide-ranging treaty will refer to the turbulent security situation faced by both countries, and include a mutual defence pact.

"There is no strategic threat to one which would not be a strategic threat to the other," pact says, with a pledge the two countries "shall assist one another, including by military means, in case of an armed attack".

While Britain and Germany already have a commitment to mutual defence as NATO members, the treaty aims to pave the way for greater defence cooperation, including operations on NATO's eastern flank.

- Train links -

German Foreign Minister Johann Wadephul accompanied Merz, meeting with his British counterpart, David Lammy.

On migration, Merz's government is expected to make a commitment to modify German law by the end of the year to criminalise the facilitation of "illegal migration".

This will include action against storage facilities used by migrant smugglers to conceal small boats intended for Channel crossings.

The two countries will also commit to improving train connections.

Last month Eurostar said it planned to launch a new route from London to Frankfurt in the early 2030s - the first such direct connection between the UK and Germany.