UK Prime Minister Keir Starmer hailed on Wednesday a defence pact with Poland, arguing the European allies faced “no greater challenge” than “Russian aggression” as he welcomed Polish counterpart Donald Tusk to Britain.

The new security treaty signed by the NATO allies aims to allow the two countries to combine their armed forces’ expertise and industrial capability, including developing and manufacturing “next-generation complex weapons”, according to the UK government.

It paves the way for large-scale joint exercises by land forces and for London and Warsaw to boost the use of uncrewed systems to reinforce NATO’s eastern flank, it said.

The agreement’s security elements will also bolster information-sharing and other cooperation to tackle organised crime and aid joint work on cyber, migration and health security.

It follows Britain’s signing of similar defence pacts with Germany and France in recent years.

Poland — an EU and NATO member that shares its eastern border with Russia, Belarus and Ukraine — also recently inked a deal in Paris to ramp up joint defence ties.

“There’s no greater challenge for either of our countries than the challenge of Russian aggression,” Starmer, flanking Tusk, said after signing the treaty at a World War II-era bunker on a former military base in northwest London.

“And we see that not just in Ukraine itself, but beyond Ukraine, impacting on our own countries,” he added, calling the treaty “a generational uplift” in the allies’ security and defence relationship.

Tusk thanked Starmer for his commitment to defending “shared values” like the rule of law, democracy and human rights, saying they were “important for us and for our nations”.

“That is the foundation of the treaty,” he noted, speaking through an interpreter.

The pair had earlier held bilateral talks at the Royal Air Force’s nearby base at Northolt.

They were expected to discuss the uptick in allegedly Russian-ordered arson attacks in London and elsewhere across Europe, as well as other malign threats, Starmer’s office said in advance.

The signing came on the same day as the head of the UK’s top-secret electronic eavesdropping agency GCHQ accused Russia of “relentlessly” targeting critical infrastructure, democratic processes, supply chains and public trust in Britain and Europe.

Delivering an inaugural annual lecture, Anne Keast-Butler detailed how Moscow has been increasing its hybrid activity against some European countries, as she urged the public and businesses to make cyber security “10 times more urgent”.

Keast-Butler — appointed GCHQ’s first woman chief in 2023 — noted her agency’s work focused on “disrupting Russia’s efforts to smuggle western tech, fending off cyber attacks, and countering reckless sabotage and assassination attempts”.