They will also discuss continued support for Ukraine, with both London and Berlin 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 to Ukraine just days after he took office in early May.

Merz's trip comes a week after a state visit to Britain by French President Emmanuel Macron, the first by an EU head of state since the UK left the bloc five years ago.

The wide-ranging new treaty between Germany and Britain will refer to the turbulent security situation that both countries face and include a mutual defence pact.

"There is no strategic threat to one which would not be a strategic threat to the other," the text is expected to say, alongside a pledge that 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 is expected to pave the way for greater defence cooperation, including in weapons development and operations on NATO's eastern flank.

The two countries already signed a landmark defence agreement in October and in May said they would begin developing strike missiles with a range of 2,000-plus kilometres (1,250 miles).

Merz and Starmer are set to announce Thursday that this system will be delivered within the next decade, and they will unveil a new agreement on boosting UK defence exports to Germany such as Boxer armoured vehicles and Typhoon jets.

- Overcoming Brexit 'trauma' -

German Foreign Minister Johann Wadephul will accompany Merz on the visit and is expected to meet his British counterpart David Lammy.

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

The treaty will include measures to improve citizens' contacts between the two countries, for example by simplifying border procedures for school trips.

The two leaders are also set to commit to doing more to tackle irregular migration, a key domestic priority for both Merz and Starmer.

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 text will also include sections on science and technology, with more cooperation promised in strategic sectors such as artificial intelligence.

It will also establish a new UK-Germany Business Forum, with several German companies poised to announce new investment in Britain on Thursday.

On the topic of transport, the two countries will commit to improving train connections.

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