LONDON (AP) -- U.K. Prime Minister Keir Starmer and German Chancellor Friedrich Merz are set to sign a treaty on Thursday pledging to tighten defense ties and step up law-enforcement cooperation against gangs that smuggle migrants across the English Channel.

The center-right German leader is in London on his first official visit to Britain since taking office in May. Starmer visited Berlin in August 2024, announcing plans for the U.K.-Germany "friendship and cooperation" treaty with Merz's predecessor, Olaf Scholz.

A priority for Starmer, who heads the center-left Labour Party, is curbing the gangs behind cross-channel people smuggling. About 37,000 people were detected crossing the English Channel from France in small boats in 2024, and more than 20,000 people made the crossing in the first six months of 2025. Dozens of people have died trying to cross.

Berlin agreed last year to make facilitating the smuggling of migrants to the U.K. a criminal offence, a move that will give law enforcements more powers to investigate the supply and storage of small boats to be used for the crossings. Merz is expected to commit to adopting the law change by the end of the year.

"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, calling the U.K. and Germany "the closest of allies."

Germany and the U.K. - Europe's largest and second-largest economies - are also expected to announce a series of investment deals.

The treaty builds on a defense pact the U.K. and Germany, two of the biggest European supporters of Ukraine, signed last year pledging closer co-operation against a growing threat from Russia. It includes a promise to come to one another's aid in case of attack.

The two leaders also are expected to agree Thursday to joint export campaigns for jointly produced equipment such as Boxer armored vehicles and Typhoon jets, and to develop a deep precision strike missile in the next decade.

Starmer has worked to improve relations with Britain's neighbors, strained by the U.K.'s acrimonious departure from the European Union in 2020. He has sought to rebuild ties strained by years of ill-tempered wrangling over Brexit terms. He has ruled out rejoining the 27-nation bloc's single market or customs union, and has been cool to the idea of a youth mobility agreement with the EU, but has sought to reduce trade barriers and to strengthen defense cooperation.

"I make no secret of the fact I very much regret to this day that Britain left the European Union," Merz told the German parliament last week. "But if they at least work together with us again in the area of foreign and security policy, then that is a very good sign. Prime Minister Keir Starmer wants that."

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Moulson reported from Berlin.